This will allow us to reduce the duplication between src/setup.py (which is currently used when ./configure --enable-editable is in use) and build/pkgs/sagelib/src/setup.py.

We also update the developer's guide on the topic of package_data.

Depends on #32899

CC: @kiwifb @dimpase @kliem @jhpalmieri @kwankyu @tobiasdiez

Component: build

Author: Matthias Koeppe

Branch/Commit: 81e9c9a

Reviewer: John Palmieri

Issue created by migration from https://trac.sagemath.org/ticket/31386