This is preparation for splitting sagelib into separate distutils packages, but a useful cleanup in any case.

As a follow-up of this ticket, as part of Meta-ticket #29705, the remaining very short src/setup.py should probably be moved to build/pkgs/sagelib/src/ (alongside a symlink src -> SAGE_ROOT/src)

Depends on #29411

CC: @kiwifb @vbraun @isuruf @dimpase @jhpalmieri @fchapoton @orlitzky

Component: refactoring

Keywords: sd109

Author: Matthias Koeppe

Branch: a56dc35

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/29702