We add two new features to find_python_sources: finding Cython extensions, and filtering by "distributions".

We remove the use of module_list, finding Cython extensions instead in the source tree. (This is prepared by #29706 and follow-up tickets by moving Extension options to directives in the source files.)

We remove OptionalExtensions as follows. We map installed packages to "distributions" (for example, tdlib -> sage-tdlib) and then filter by distribution.

Follow-up tickets:

• as part of Meta-ticket Meta-ticket: Modularize sagelib into separate distributions (pip-installable packages) sagemath-... for Sage 10.x #29705, we will make these "distributions" actually separate distutils packages that provide namespace packages (Modify find_python_sources to support modularization of sagelib by native namespace packages (PEP 420) #28925)
• remove the file module_list.py (not done on this ticket to avoid possible merge conflicts), deprecate OptionalExtension (which may be in use by user packages?)

CC: @kiwifb @isuruf @videlec @dcoudert @dimpase @kliem @vbraun

Component: refactoring

Author: Matthias Koeppe

Branch/Commit: 55c3fbc

Reviewer: Jonathan Kliem

Issue created by migration from https://trac.sagemath.org/ticket/29701