Make submodule checks work under Python 3.

My shot at fixing gh-3385.

- Ditch the imports from utils.submodule
- Move the `is_repo` check up: distro packaging will often not be running from a git repository, in which case packagers won't need to do anything. If we're not in a git repository, there's nothing sensible we can do about submodules anyway.
- Clearly label where packagers can patch to disable the submodule tests completely.