Cherry-picking virtual dependencies (#35322)

This PR makes it possible to select only a subset of virtual dependencies from a spec that _may_ provide more. To select providers, a syntax to specify edge attributes is introduced:
```
hdf5 ^[virtuals=mpi] mpich
```
With that syntax we can concretize specs like:
```console
$ spack spec strumpack ^[virtuals=mpi] intel-parallel-studio+mkl ^[virtuals=lapack] openblas
```

On `develop` this would currently fail with:
```console
$ spack spec strumpack ^intel-parallel-studio+mkl ^openblas
==> Error: Spec cannot include multiple providers for virtual 'blas'
    Requested 'intel-parallel-studio' and 'openblas'
```

In package recipes, virtual specs that are declared in the same `provides` directive need to be provided _together_. This means that e.g. `openblas`, which has:
```python
provides("blas", "lapack")
```
needs to provide both `lapack` and `blas` when requested to provide at least one of them.

## Additional notes

This capability is needed to model compilers. Assuming that languages are treated like virtual dependencies, we might want e.g. to use LLVM to compile C/C++ and Gnu GCC to compile Fortran. This can be accomplished by the following[^1]:
```
hdf5 ^[virtuals=c,cxx] llvm ^[virtuals=fortran] gcc
```

[^1]: We plan to add some syntactic sugar around this syntax, and reuse the `%` sigil to avoid having a lot of boilerplate around compilers.

Modifications:
- [x] Add syntax to interact with edge attributes from spec literals
- [x] Add concretization logic to be able to cherry-pick virtual dependencies
- [x] Extend semantic of the `provides` directive to express when virtuals need to be provided together
- [x] Add unit-tests and documentation

5 files changed, 43 insertions, 5 deletions

diff --git a/var/spack/repos/builtin.mock/packages/intel-parallel-studio/package.py b/var/spack/repos/builtin.mock/packages/intel-parallel-studio/package.py
new file mode 100644
index 0000000000..1ec5cf6932
--- /dev/null
+++ b/var/spack/repos/builtin.mock/packages/intel-parallel-studio/package.py
@@ -0,0 +1,19 @@
+# Copyright 2013-2023 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+from spack.package import *
+
+
+class IntelParallelStudio(Package):
+    """Intel Parallel Studio."""
+
+    homepage = "https://software.intel.com/en-us/intel-parallel-studio-xe"
+    url = "http://tec/16225/parallel_studio_xe_2020_cluster_edition.tgz"
+
+    version("cluster.2020.0", sha256="b1d3e3e425b2e44a06760ff173104bdf")
+
+    provides("mpi@:3")
+    provides("scalapack")
+    provides("blas", "lapack")
diff --git a/var/spack/repos/builtin.mock/packages/low-priority-provider/package.py b/var/spack/repos/builtin.mock/packages/low-priority-provider/package.py
index 5b7bfc03c1..940dea3daf 100644
--- a/var/spack/repos/builtin.mock/packages/low-priority-provider/package.py
+++ b/var/spack/repos/builtin.mock/packages/low-priority-provider/package.py
@@ -14,5 +14,5 @@ class LowPriorityProvider(Package):
 
     version("1.0", md5="0123456789abcdef0123456789abcdef")
 
-    provides("lapack")
-    provides("mpi")
+    # A low priority provider that provides both these specs together
+    provides("mpi", "lapack")
diff --git a/var/spack/repos/builtin.mock/packages/many-virtual-consumer/package.py b/var/spack/repos/builtin.mock/packages/many-virtual-consumer/package.py
index 070adf60bc..087cfb77cc 100644
--- a/var/spack/repos/builtin.mock/packages/many-virtual-consumer/package.py
+++ b/var/spack/repos/builtin.mock/packages/many-virtual-consumer/package.py
@@ -19,4 +19,4 @@ class ManyVirtualConsumer(Package):
     # This directive is an example of imposing a constraint on a
     # dependency is that dependency is in the DAG. This pattern
     # is mainly used with virtual providers.
-    depends_on("low-priority-provider@1.0", when="^low-priority-provider")
+    depends_on("low-priority-provider@1.0", when="^[virtuals=mpi,lapack] low-priority-provider")
diff --git a/var/spack/repos/builtin.mock/packages/netlib-scalapack/package.py b/var/spack/repos/builtin.mock/packages/netlib-scalapack/package.py
new file mode 100644
index 0000000000..fe5d7f90a1
--- /dev/null
+++ b/var/spack/repos/builtin.mock/packages/netlib-scalapack/package.py
@@ -0,0 +1,20 @@
+# Copyright 2013-2023 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+
+from spack.package import *
+
+
+class NetlibScalapack(Package):
+    homepage = "http://www.netlib.org/scalapack/"
+    url = "http://www.netlib.org/scalapack/scalapack-2.1.0.tgz"
+
+    version("2.1.0", "b1d3e3e425b2e44a06760ff173104bdf")
+
+    provides("scalapack")
+
+    depends_on("mpi")
+    depends_on("lapack")
+    depends_on("blas")
diff --git a/var/spack/repos/builtin.mock/packages/openblas-with-lapack/package.py b/var/spack/repos/builtin.mock/packages/openblas-with-lapack/package.py
index 0156085877..1273b70def 100644
--- a/var/spack/repos/builtin.mock/packages/openblas-with-lapack/package.py
+++ b/var/spack/repos/builtin.mock/packages/openblas-with-lapack/package.py
@@ -14,5 +14,4 @@ class OpenblasWithLapack(Package):
 
     version("0.2.15", md5="b1190f3d3471685f17cfd1ec1d252ac9")
 
-    provides("lapack")
-    provides("blas")
+    provides("lapack", "blas")