Switch from distutils to setuptools + implicit namespace package to fix namespace package issues

This merge request is a superset of !23 (merged) which was reverted, but also removes the pkg_resources namespace registration in ligo/__init__.py` in favor of an implicit namespace.

Also updates spec.in and docs stage in CI to support these changes.

I can confirm that I don't see any of the issues described in #16 (closed) running the test suite in Bullseye, RL8 or pip. This also resolves the issue described in https://git.ligo.org/kipp.cannon/python-ligo-lw/-/issues/16#note_615980 with needing to import pkg_resources prior to importing ligo-lw if you have multiple versions of ligo-lw installed.

Closes #16 (closed).