bugfix: 'submodules' attribute should imply git fetch

- 'submodules' wasn't properly declared as an optional attribute for git
  fetcher

- add it and add a test.

var/spack/repos/builtin.mock/packages/git-url-top-level/package.py

@@ -39,6 +39,7 @@ class GitUrlTopLevel(Package):
 
     # These resolve to git fetchers
     version('develop', branch='develop')
+    version('submodules', submodules=True)
     version('3.4', commit='abc34')
     version('3.3', branch='releases/v3.3', commit='abc33')
     version('3.2', branch='releases/v3.2')