May I suggest using the submodule_search_locations when it is a namespace package.
I think that it would be correct to use submodule_search_locations because PEP 451 mention this:

"submodule_search_locations" can be deduced from loader.is_package() and from os.path.dirname(location) if location is a filename.

I think the return would look something like the one done for the is_package condition.

 if spec.loader is None and spec.submodule_search_locations:
        return None, os.path.dirname(str(spec.submodule_search_locations)), ("", "", _PKG_DIRECTORY)

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While this fix appears to solve the top-level namespace package, it seems to fail on solving the nested namespace packages, i.e. from foo.bar.baz import qux, where both /foo and /foo/bar are missing __init__.py.