Employ native namespace packages for pmxbot and pmxbot.web.

[jaraco]

• Switch to a PEP 420 native namespace package. Fixes pmxbot uses deprecated pkg_resources namespace packages #108.
• Enable importlib import mode to honor native namespace package.
• Try importing from . instead of tests.functional. Fails with unknown location for tests.functional.
• Move functional harness to a separate module to avoid import errors.
• Employ bugfix in Fix duplicated imports with importlib mode and doctest-modules pytest-dev/pytest#11148.
• Additionally employ bugfix in Add child modules as attributes of parent modules. pytest-dev/pytest#10338.