use Distribution object to resolve packages

[thrau]

Motivation

The previous package finding mechanism wasn't resolving namespace packages correctly. There is a special PEP420PackageFinder that we could use, but while investigating, I noticed that the setuptools Distribution object already resolves packages correctly based on the tool.setuptools.package.find config in either the setup.cfg or pyproject.toml. Using this instead instead of setuptools.find_package directly this also solves the outstanding issue of correctly passing where, exclude, and include parameters to the finder.

We still require the source path (where) to load all modules, code which we already introduced in #17.

Changes

• plux now resolves packages from the Distribution object instead of calling setuptools.find_package manually.