Meta-ticket: Replace use of OptionalExtension by namespace packages

[mkoeppe]

Currently, a user would install, for example, the optional package tdlib and then rebuild sagelib so that the OptionalExtension sage.graphs.graph_decompositions.tdlib is built and installed.

With this ticket, the user would instead install a new optional package sage_tdlib (which has tdlib as a dependency); this installs the Extension sage.graphs.graph_decompositions.tdlib (as a namespace package).

Preparation:

• #28925 Modify find_python_sources, clean_stale_files to support modularization of sagelib by native namespace packages (PEP 420)
• Remove empty __init__.py files to convert packages to native namespace packages
• #29702 Move all code from src/setup.py to sage_setup

Tickets for individual OptionalExtensions (see src/module_list.py):

• sage.graphs.mcqd
• sage.graphs.bliss
• sage.graphs.graph_decompositions.tdlib (used as an example in #28925)
• sage.interfaces.primecount (how come this is not in sage.libs, where primecount.pxd lives?)
• sage.libs.coxeter3.coxeter
• sage.libs.fes
• sage.libs.sirocco
• sage.libs.meataxe, sage.matrix.matrix_gfpn_dense