This ticket changes the following packages to namespace packages (by removing __init__.py):

• sage.matrix (this allows the new distribution sage-meataxe introduced in Modularization of sagelib: Break out a separate package sagemath-meataxe #30151 to add the module sage.matrix.matrix_gfpn_dense) to the package)
• sage.libs (for .meataxe - Modularization of sagelib: Break out a separate package sagemath-meataxe #30151, .sirocco, .coexter3, .fes)
• sage.interfaces (for .primecount)
• sage.graphs (for .bliss, .mcqd)
• sage.graphs.graph_decompositions (for .tdlib -> sage-tdlib, Modularization of sagelib: Create package sage-tdlib #29864.)
• sage.numerical, sage.numerical.backends. (This is preparation for later work, in sage_setup: Modify clean_stale_files to support out-of-tree namespace packages #30152.)

A complication lies in the behavior of the Python import machinery:
setup.py puts the source path in front of setup.py because sage_setup uses sage.env and is_package_or_sage_namespace_package_dir (#33033).
But when an old version of sage that is an ordinary package is installed already, the source will not shadow it.

To avoid this complication, in this ticket we do not yet remove src/sage/__init__.py. That is done in #34187.

CC: @tobiasdiez @kwankyu

Component: refactoring

Author: Matthias Koeppe

Branch/Commit: 39aa2f1

Reviewer: Kwankyu Lee

Issue created by migration from https://trac.sagemath.org/ticket/33011