Package-level submodule attribute: support explicit versions (#30085)

var/spack/repos/builtin.mock/packages/git-test/package.py

@@ -9,6 +9,8 @@
 class GitTest(Package):
     """Mock package that uses git for fetching."""
     homepage = "http://www.git-fetch-example.com"
+    # To be set by test
+    git = None
 
     submodules = True