gc: restrict to specific specs (#46790)

`spack gc` has so far been a global or environment-specific thing.
This adds the ability to restrict garbage collection to specific specs,
e.g. if you *just* want to get rid of all your unused python installations,
you could write:

```console
spack gc python
```

- [x] add `constraint` arg to `spack gc`
- [x] add a simple test

Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>

lib/spack/spack/cmd/gc.py

@@ -41,7 +41,7 @@ def setup_parser(subparser):
         help="do not remove installed build-only dependencies of roots\n"
         "(default is to keep only link & run dependencies)",
     )
-    spack.cmd.common.arguments.add_common_arguments(subparser, ["yes_to_all"])
+    spack.cmd.common.arguments.add_common_arguments(subparser, ["yes_to_all", "constraint"])
 
 
 def roots_from_environments(args, active_env):
@@ -97,6 +97,12 @@ def gc(parser, args):
             root_hashes = None
 
         specs = spack.store.STORE.db.unused_specs(root_hashes=root_hashes, deptype=deptype)
+
+        # limit search to constraint specs if provided
+        if args.constraint:
+            hashes = set(spec.dag_hash() for spec in args.specs())
+            specs = [spec for spec in specs if spec.dag_hash() in hashes]
+
         if not specs:
             tty.msg("There are no unused specs. Spack's store is clean.")
             return

lib/spack/spack/test/cmd/gc.py

@@ -35,6 +35,22 @@ def test_gc_with_build_dependency(mutable_database):
     assert "There are no unused specs." in gc("-y")
 
 
+@pytest.mark.db
+def test_gc_with_constraints(mutable_database):
+    s_cmake1 = spack.spec.Spec("simple-inheritance ^cmake@3.4.3").concretized()
+    s_cmake2 = spack.spec.Spec("simple-inheritance ^cmake@3.23.1").concretized()
+    PackageInstaller([s_cmake1.package], explicit=True, fake=True).install()
+    PackageInstaller([s_cmake2.package], explicit=True, fake=True).install()
+
+    assert "There are no unused specs." in gc("python")
+
+    assert "Successfully uninstalled cmake@3.4.3" in gc("-y", "cmake@3.4.3")
+    assert "There are no unused specs." in gc("-y", "cmake@3.4.3")
+
+    assert "Successfully uninstalled cmake" in gc("-y", "cmake@3.23.1")
+    assert "There are no unused specs." in gc("-y", "cmake")
+
+
 @pytest.mark.db
 def test_gc_with_environment(mutable_database, mutable_mock_env_path):
     s = spack.spec.Spec("simple-inheritance")