Makefile generator for parallel spack install of environments (#30254)

`make` solves a lot of headaches that would otherwise have to be implemented in Spack:

1. Parallelism over packages through multiple `spack install` processes
2. Orderly output of parallel package installs thanks to `make --sync-output=recurse` or `make -Orecurse` (works well in GNU Make 4.3; macOS is unfortunately on a 16 years old 3.x version, but it's one `spack install gmake` away...)
3. Shared jobserver across packages, which means a single `-j` to rule them all, instead of manually finding a balance between `#spack install processes` & `#jobs per package` (See #30302).

This pr adds the `spack env depfile` command that generates a Makefile with dag hashes as
targets, and dag hashes of dependencies as prerequisites, and a command
along the lines of `spack install --only=packages /hash` to just install
a single package.

It exposes two convenient phony targets: `all`, `fetch-all`. The former installs the environment, the latter just fetches all sources. So one can either use `make all -j16` directly or run `make fetch-all -j16` on a login node and `make all -j16` on a compute node. 

Example:

```yaml
spack:
  specs: [perl]
  view: false
```

running

```
$ spack -e . env depfile --make-target-prefix env | tee Makefile
```
generates

```Makefile
SPACK ?= spack

.PHONY: env/all env/fetch-all env/clean

env/all: env/env

env/fetch-all: env/fetch

env/env: env/.install/cdqldivylyxocqymwnfzmzc5sx2zwvww
	@touch $@

env/fetch: env/.fetch/cdqldivylyxocqymwnfzmzc5sx2zwvww env/.fetch/gv5kin2xnn33uxyfte6k4a3bynhmtxze env/.fetch/cuymc7e5gupwyu7vza5d4vrbuslk277p env/.fetch/7vangk4jvsdgw6u6oe6ob63pyjl5cbgk env/.fetch/hyb7ehxxyqqp2hiw56bzm5ampkw6cxws env/.fetch/yfz2agazed7ohevqvnrmm7jfkmsgwjao env/.fetch/73t7ndb5w72hrat5hsax4caox2sgumzu env/.fetch/trvdyncxzfozxofpm3cwgq4vecpxixzs env/.fetch/sbzszb7v557ohyd6c2ekirx2t3ctxfxp env/.fetch/c4go4gxlcznh5p5nklpjm644epuh3pzc
	@touch $@

env/dirs:
	@mkdir -p env/.fetch env/.install

env/.fetch/%: | env/dirs
	$(info Fetching $(SPEC))
	$(SPACK) -e '/tmp/tmp.7PHPSIRACv' fetch $(SPACK_FETCH_FLAGS) /$(notdir $@) && touch $@

env/.install/%: env/.fetch/%
	$(info Installing $(SPEC))
	+$(SPACK) -e '/tmp/tmp.7PHPSIRACv' install $(SPACK_INSTALL_FLAGS) --only-concrete --only=package --no-add /$(notdir $@) && touch $@

# Set the human-readable spec for each target
env/%/cdqldivylyxocqymwnfzmzc5sx2zwvww: SPEC = perl@5.34.1%gcc@10.3.0+cpanm+shared+threads arch=linux-ubuntu20.04-zen2
env/%/gv5kin2xnn33uxyfte6k4a3bynhmtxze: SPEC = berkeley-db@18.1.40%gcc@10.3.0+cxx~docs+stl patches=b231fcc arch=linux-ubuntu20.04-zen2
env/%/cuymc7e5gupwyu7vza5d4vrbuslk277p: SPEC = bzip2@1.0.8%gcc@10.3.0~debug~pic+shared arch=linux-ubuntu20.04-zen2
env/%/7vangk4jvsdgw6u6oe6ob63pyjl5cbgk: SPEC = diffutils@3.8%gcc@10.3.0 arch=linux-ubuntu20.04-zen2
env/%/hyb7ehxxyqqp2hiw56bzm5ampkw6cxws: SPEC = libiconv@1.16%gcc@10.3.0 libs=shared,static arch=linux-ubuntu20.04-zen2
env/%/yfz2agazed7ohevqvnrmm7jfkmsgwjao: SPEC = gdbm@1.19%gcc@10.3.0 arch=linux-ubuntu20.04-zen2
env/%/73t7ndb5w72hrat5hsax4caox2sgumzu: SPEC = readline@8.1%gcc@10.3.0 arch=linux-ubuntu20.04-zen2
env/%/trvdyncxzfozxofpm3cwgq4vecpxixzs: SPEC = ncurses@6.2%gcc@10.3.0~symlinks+termlib abi=none arch=linux-ubuntu20.04-zen2
env/%/sbzszb7v557ohyd6c2ekirx2t3ctxfxp: SPEC = pkgconf@1.8.0%gcc@10.3.0 arch=linux-ubuntu20.04-zen2
env/%/c4go4gxlcznh5p5nklpjm644epuh3pzc: SPEC = zlib@1.2.12%gcc@10.3.0+optimize+pic+shared patches=0d38234 arch=linux-ubuntu20.04-zen2

# Install dependencies
env/.install/cdqldivylyxocqymwnfzmzc5sx2zwvww: env/.install/gv5kin2xnn33uxyfte6k4a3bynhmtxze env/.install/cuymc7e5gupwyu7vza5d4vrbuslk277p env/.install/yfz2agazed7ohevqvnrmm7jfkmsgwjao env/.install/c4go4gxlcznh5p5nklpjm644epuh3pzc
env/.install/cuymc7e5gupwyu7vza5d4vrbuslk277p: env/.install/7vangk4jvsdgw6u6oe6ob63pyjl5cbgk
env/.install/7vangk4jvsdgw6u6oe6ob63pyjl5cbgk: env/.install/hyb7ehxxyqqp2hiw56bzm5ampkw6cxws
env/.install/yfz2agazed7ohevqvnrmm7jfkmsgwjao: env/.install/73t7ndb5w72hrat5hsax4caox2sgumzu
env/.install/73t7ndb5w72hrat5hsax4caox2sgumzu: env/.install/trvdyncxzfozxofpm3cwgq4vecpxixzs
env/.install/trvdyncxzfozxofpm3cwgq4vecpxixzs: env/.install/sbzszb7v557ohyd6c2ekirx2t3ctxfxp

env/clean:
	rm -f -- env/env env/fetch env/.fetch/cdqldivylyxocqymwnfzmzc5sx2zwvww env/.fetch/gv5kin2xnn33uxyfte6k4a3bynhmtxze env/.fetch/cuymc7e5gupwyu7vza5d4vrbuslk277p env/.fetch/7vangk4jvsdgw6u6oe6ob63pyjl5cbgk env/.fetch/hyb7ehxxyqqp2hiw56bzm5ampkw6cxws env/.fetch/yfz2agazed7ohevqvnrmm7jfkmsgwjao env/.fetch/73t7ndb5w72hrat5hsax4caox2sgumzu env/.fetch/trvdyncxzfozxofpm3cwgq4vecpxixzs env/.fetch/sbzszb7v557ohyd6c2ekirx2t3ctxfxp env/.fetch/c4go4gxlcznh5p5nklpjm644epuh3pzc env/.install/cdqldivylyxocqymwnfzmzc5sx2zwvww env/.install/gv5kin2xnn33uxyfte6k4a3bynhmtxze env/.install/cuymc7e5gupwyu7vza5d4vrbuslk277p env/.install/7vangk4jvsdgw6u6oe6ob63pyjl5cbgk env/.install/hyb7ehxxyqqp2hiw56bzm5ampkw6cxws env/.install/yfz2agazed7ohevqvnrmm7jfkmsgwjao env/.install/73t7ndb5w72hrat5hsax4caox2sgumzu env/.install/trvdyncxzfozxofpm3cwgq4vecpxixzs env/.install/sbzszb7v557ohyd6c2ekirx2t3ctxfxp env/.install/c4go4gxlcznh5p5nklpjm644epuh3pzc
```

Then with `make -O` you get very nice orderly output when packages are built in parallel:
```console
$ make -Orecurse -j16
spack -e . install --only-concrete --only=package /c4go4gxlcznh5p5nklpjm644epuh3pzc && touch c4go4gxlcznh5p5nklpjm644epuh3pzc
==> Installing zlib-1.2.12-c4go4gxlcznh5p5nklpjm644epuh3pzc
...
  Fetch: 0.00s.  Build: 0.88s.  Total: 0.88s.
[+] /tmp/tmp.b1eTyAOe85/store/linux-ubuntu20.04-zen2/gcc-10.3.0/zlib-1.2.12-c4go4gxlcznh5p5nklpjm644epuh3pzc
spack -e . install --only-concrete --only=package /sbzszb7v557ohyd6c2ekirx2t3ctxfxp && touch sbzszb7v557ohyd6c2ekirx2t3ctxfxp
==> Installing pkgconf-1.8.0-sbzszb7v557ohyd6c2ekirx2t3ctxfxp
...
  Fetch: 0.00s.  Build: 3.96s.  Total: 3.96s.
[+] /tmp/tmp.b1eTyAOe85/store/linux-ubuntu20.04-zen2/gcc-10.3.0/pkgconf-1.8.0-sbzszb7v557ohyd6c2ekirx2t3ctxfxp
```

For Perl, at least for me, using `make -j16` versus `spack -e . install -j16` speeds up the builds from 3m32.623s to 2m22.775s, as some configure scripts run in parallel.

Another nice feature is you can do Makefile "metaprogramming" and depend on packages built by Spack. This example fetches all sources (in parallel) first, print a message, and only then build packages (in parallel).

```Makefile
SPACK ?= spack

.PHONY: env

all: env

spack.lock: spack.yaml
	$(SPACK) -e . concretize -f

env.mk: spack.lock
	$(SPACK) -e . env depfile -o $@ --make-target-prefix spack

fetch: spack/fetch
	@echo Fetched all packages && touch $@

env: fetch spack/env
	@echo This executes after the environment has been installed

clean:
	rm -rf spack/ env.mk spack.lock

ifeq (,$(filter clean,$(MAKECMDGOALS)))
include env.mk
endif
```

1 files changed, 15 insertions, 3 deletions

diff --git a/lib/spack/docs/conf.py b/lib/spack/docs/conf.py
index 5455aa0f28..48746d149e 100644
--- a/lib/spack/docs/conf.py
+++ b/lib/spack/docs/conf.py
@@ -23,7 +23,10 @@ import subprocess
 import sys
 from glob import glob
 
+from docutils.statemachine import StringList
+from sphinx.domains.python import PythonDomain
 from sphinx.ext.apidoc import main as sphinx_apidoc
+from sphinx.parsers import RSTParser
 
 # -- Spack customizations -----------------------------------------------------
 # If extensions (or modules to document with autodoc) are in another directory,
@@ -82,9 +85,6 @@ todo_include_todos = True
 #
 # Disable duplicate cross-reference warnings.
 #
-from sphinx.domains.python import PythonDomain
-
-
 class PatchedPythonDomain(PythonDomain):
     def resolve_xref(self, env, fromdocname, builder, typ, target, node, contnode):
         if 'refspecific' in node:
@@ -92,8 +92,20 @@ class PatchedPythonDomain(PythonDomain):
         return super(PatchedPythonDomain, self).resolve_xref(
             env, fromdocname, builder, typ, target, node, contnode)
 
+#
+# Disable tabs to space expansion in code blocks
+# since Makefiles require tabs.
+#
+class NoTabExpansionRSTParser(RSTParser):
+    def parse(self, inputstring, document):
+        if isinstance(inputstring, str):
+            lines = inputstring.splitlines()
+            inputstring = StringList(lines, document.current_source)
+        super().parse(inputstring, document)
+
 def setup(sphinx):
     sphinx.add_domain(PatchedPythonDomain, override=True)
+    sphinx.add_source_parser(NoTabExpansionRSTParser, override=True)
 
 # -- General configuration -----------------------------------------------------