cmd: add `spack mark` command (#16662)

This adds a new `mark` command that can be used to mark packages as either
explicitly or implicitly installed. Apart from fixing the package
database after installing a dependency manually, it can be used to
implement upgrade workflows as outlined in #13385.

The following commands demonstrate how the `mark` and `gc` commands can be
used to only keep the current version of a package installed:
```console
$ spack install pkgA
$ spack install pkgB
$ git pull # Imagine new versions for pkgA and/or pkgB are introduced
$ spack mark -i -a
$ spack install pkgA
$ spack install pkgB
$ spack gc
```

If there is no new version for a package, `install` will simply mark it as
explicitly installed and `gc` will not remove it.

Co-authored-by: Greg Becker <becker33@llnl.gov>

1 files changed, 10 insertions, 1 deletions

diff --git a/share/spack/spack-completion.bash b/share/spack/spack-completion.bash
index 969a0898fe..bf44039ad9 100755
--- a/share/spack/spack-completion.bash
+++ b/share/spack/spack-completion.bash
@@ -320,7 +320,7 @@ _spack() {
     then
         SPACK_COMPREPLY="-h --help -H --all-help --color -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 -p --profile --sorted-profile --lines -v --verbose --stacktrace -V --version --print-shell-vars"
     else
-        SPACK_COMPREPLY="activate add arch blame build-env buildcache cd checksum ci clean clone commands compiler compilers concretize config containerize create deactivate debug dependencies dependents deprecate dev-build develop docs edit env extensions external fetch find flake8 gc gpg graph help info install license list load location log-parse maintainers mirror module patch pkg providers pydoc python reindex remove rm repo resource restage setup solve spec stage test test-env tutorial undevelop uninstall unit-test unload url verify versions view"
+        SPACK_COMPREPLY="activate add arch blame build-env buildcache cd checksum ci clean clone commands compiler compilers concretize config containerize create deactivate debug dependencies dependents deprecate dev-build develop docs edit env extensions external fetch find flake8 gc gpg graph help info install license list load location log-parse maintainers mark mirror module patch pkg providers pydoc python reindex remove rm repo resource restage setup solve spec stage test test-env tutorial undevelop uninstall unit-test unload url verify versions view"
     fi
 }
 
@@ -1088,6 +1088,15 @@ _spack_maintainers() {
     fi
 }
 
+_spack_mark() {
+    if $list_options
+    then
+        SPACK_COMPREPLY="-h --help -a --all -e --explicit -i --implicit"
+    else
+        _installed_packages
+    fi
+}
+
 _spack_mirror() {
     if $list_options
     then