fix: use native namespace to avoid pkg_resources warnings

[andrewsg]

Fixes#1000 🦕

[parthea]

Sorry I accidentally clicked the close button while I was typing..

For other google-cloud-* libraries, we'll follow the same change in googleapis/google-auth-library-python#1205 which includes switching to find_namespace_packages as described in https://packaging.python.org/en/latest/guides/packaging-namespace-packages/#native-namespace-packages

[andrewsg]

@parthea Thank you very much. PTAL

[parthea]

Please also remove namespace_packages= in setup.py and add a packaging test similar to https://github.com/googleapis/google-auth-library-python/pull/1205/files

[andrewsg]

@parthea Test will be added after dependencies are updated, see googleapis/google-cloud-python#15980 . Took care of the setup.py updates. PTAL