This is preparation for #30371 and #28925.

src/setup.py will be modified in #30371 for development installs.

build/pkgs/sagelib/src/setup.py will be modified in #28925 for namespace installs.

CC: @tobiasdiez

Component: build

Author: Matthias Koeppe

Branch/Commit: 4f56517

Reviewer: Tobias Diez

Issue created by migration from https://trac.sagemath.org/ticket/30779