Build: Simplify setup.cfg not to include every module

[glensc]

• https://setuptools.pypa.io/en/latest/userguide/package_discovery.html
• https://setuptools.pypa.io/en/latest/userguide/declarative_config.html

Extracted from #1894

NOTE: Doesn't work with pipx installing from pr:

It worked locally (python -m build) but pipx for pr install (pipx install --suffix=@1894 --force git+https://github.com/Taxel/PlexTraktSync@refs/pull/1894/head) failed to find files from subdirectories.

• Simplify .dockerignore without need to list modules #1894