use native support for namespace packages

[tswast]

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

[tseaver]

Something in my memory is niggling at me about doing this change incrementally: we need to test that a native namespace google-cloud-bigquery can be installed safely alongside our current mixed-mode google-auth / google-resumable-media / google-api-core / google-cloud-core.

[meredithslota]

The GAPIC libraries dropped pkg_resources in Oct. 2022 here: googleapis/gapic-generator-python#1471 so we should consider doing the same for consistency. I believe @chalmerlowe was interested in working on this across the libraries that still need this done.