feat: Introduce compatibility with native namespace packages

[parthea]

Similar to googleapis/google-auth-library-python#1205
Also see https://packaging.python.org/en/latest/guides/packaging-namespace-packages/#native-namespace-packages

[parthea]

Adding do not merge as there is an upcoming release freeze. This should be merged after the release freeze ends on November 29th.

[parthea]

Presubmit is failing with

nox > coverage report --show-missing --fail-under=99
No source for code: '/tmp/pytest-of-root/pytest-0/test_namespace_package_compat0/google/cloud/othermod.py'.
nox > Command coverage report --show-missing --fail-under=99 failed with exit code 1
nox > Session cover failed.

[harshachinta]

@parthea
Quick followup based on googleapis/python-spanner-sqlalchemy#375 (comment)

ModuleNotFoundError: No module named 'google.cloud.spanner_v1'

Will customers see some issue with latest release similar to how sqlalchemy repo was facing? Can you please help understand this change?

[parthea]

@harshachinta , Please see my response in googleapis/python-spanner-sqlalchemy#375 (comment)