use native support for namespace packages #313
Description
Per: googleapis/google-auth-library-python#595 (comment)
It looks like different Google API packages have different approaches to using or not using pkg_resources here; e.g. https://github.com/googleapis/python-cloudbuild uses native NS packages and https://github.com/googleapis/python-bigquery uses pkg_resources with a pkgutil fallback.
Now that Python 2.7 and 3.5 support has been dropped, we may be able to follow the example in python-cloudbuild