Add subscript notation to packages (#48467)

This PR allows using the subscript notation directly on packages. The
intent is to reduce the boilerplate needed to retrieve package
properties from nodes other than root.

lib/spack/spack/package_base.py

@@ -767,6 +767,9 @@ def __init__(self, spec):
         self.win_rpath = fsys.WindowsSimulatedRPath(self)
         super().__init__()
 
+    def __getitem__(self, key: str) -> "PackageBase":
+        return self.spec[key].package
+
     @classmethod
     def dependency_names(cls):
         return _subkeys(cls.dependencies)

lib/spack/spack/test/package_class.py

@@ -285,3 +285,16 @@ def compilers(compiler, arch_spec):
     error = capfd.readouterr()[1]
     assert "Skipping tests for package" in error
     assert "test requires missing compiler" in error
+
+
+def test_package_subscript(default_mock_concretization):
+    """Tests that we can use the subscript notation on packages, and that it returns a package"""
+    root = default_mock_concretization("mpileaks")
+    root_pkg = root.package
+
+    # Subscript of a virtual
+    assert isinstance(root_pkg["mpi"], spack.package_base.PackageBase)
+
+    # Subscript on concrete
+    for d in root.traverse():
+        assert isinstance(root_pkg[d.name], spack.package_base.PackageBase)