This simplifies src/module_list.py a lot. This is preparation for splitting sagelib into separate distutils packages.

Follow-up tickets take care of the remaining packages; in the end, in #29701, src/module_list.py will no longer be used, and in another follow-up ticket, it will disappear completely:

• Move Extension options from src/module_list.py to "distutils:" directives in the individual files (part 2 - OptionalExtensions) #29720: Move Extension options ... (part 2 - OptionalExtensions)
• Move Extension options from src/module_list.py to "distutils:" directives in the individual files (part 3: Get rid of uname_specific) #29785: Move Extension options ... (part 3: Get rid of uname_specific)
• Move Extension options from src/module_list.py to "distutils:" directives in the individual files (part 4: sage.rings) #29786: Move Extension options ... (part 4: sage.rings)
• Move Extension options from src/module_list.py to "distutils:" directives in the individual files (part 5: sage.graphs) #29790: Move Extension options ... (part 5: sage.graphs)
• Move Extension options from src/module_list.py to "distutils:" directives in the individual files (part 6: last) #29791: Move Extension options ... (part 6: last)
• coxeter3: Add spkg-configure.m4, use coxeter/ prefix for includes, move OptionalExtension options to .pyx file directives #29721: takes care of sage.libs.coxeter3
• Replace use of module_list and OptionalExtension by extending find_python_sources #29701: takes care of sage.graphs.graph_decompositions

CC: @kiwifb @dimpase @jhpalmieri @tscrim @kliem @roed314 @videlec @vbraun

Component: refactoring

Keywords: sd109

Author: Matthias Koeppe

Branch/Commit: 1bfe30d

Reviewer: Jonathan Kliem, John Palmieri

Issue created by migration from https://trac.sagemath.org/ticket/29706