New command, `spack change`, to change existing env specs (#31995)

If you have an environment like

```
$ cat spack.yaml
spack:
  specs: [openmpi@4.1.0+cuda]
```

this PR provides a new command `spack change` that you can use to adjust environment specs from the command line:

```
$ spack change openmpi~cuda
$ cat spack.yaml
spack:
  specs: [openmpi@4.1.0~cuda]
```

in other words, this allows you to tweak the details of environment specs from the command line.

Notes:

* This is only allowed for environments that do not define matrices
  * This is possible but not anticipated to be needed immediately
  * If this were done, it should probably only be done for "named"/not-anonymous specs (i.e. we can change `openmpi+cuda` but not spec like `+cuda` or `@4.0.1~cuda`)

1 files changed, 10 insertions, 1 deletions

diff --git a/share/spack/spack-completion.bash b/share/spack/spack-completion.bash
index 4cfde14e7a..58e94c036a 100755
--- a/share/spack/spack-completion.bash
+++ b/share/spack/spack-completion.bash
@@ -337,7 +337,7 @@ _spack() {
     then
         SPACK_COMPREPLY="-h --help -H --all-help --color -c --config -C --config-scope -d --debug --timestamp --pdb -e --env -D --env-dir -E --no-env --use-env-repo -k --insecure -l --enable-locks -L --disable-locks -m --mock -b --bootstrap -p --profile --sorted-profile --lines -v --verbose --stacktrace -V --version --print-shell-vars"
     else
-        SPACK_COMPREPLY="activate add arch audit blame bootstrap build-env buildcache cd checksum ci clean clone commands compiler compilers concretize config containerize create deactivate debug dependencies dependents deprecate dev-build develop diff docs edit env extensions external fetch find gc gpg graph help info install license list load location log-parse maintainers make-installer mark mirror module patch pkg providers pydoc python reindex remove rm repo resource restage solve spec stage style tags test test-env tutorial undevelop uninstall unit-test unload url verify versions view"
+        SPACK_COMPREPLY="activate add arch audit blame bootstrap build-env buildcache cd change checksum ci clean clone commands compiler compilers concretize config containerize create deactivate debug dependencies dependents deprecate dev-build develop diff docs edit env extensions external fetch find gc gpg graph help info install license list load location log-parse maintainers make-installer mark mirror module patch pkg providers pydoc python reindex remove rm repo resource restage solve spec stage style tags test test-env tutorial undevelop uninstall unit-test unload url verify versions view"
     fi
 }
 
@@ -585,6 +585,15 @@ _spack_cd() {
     fi
 }
 
+_spack_change() {
+    if $list_options
+    then
+        SPACK_COMPREPLY="-h --help -l --list-name --match-spec -a --all"
+    else
+        _all_packages
+    fi
+}
+
 _spack_checksum() {
     if $list_options
     then