Redesign of #31396 that does not need to put a huge wheel on PyPI

• huge wheel sage_root_with_var_tmp_symlink and wheels for all Python package linking to libraries there are published on https://github.com/sagemath/sage-wheels/releases (no size limit)

• sage_conf (sdist and wheel) declares install_requires that @ to the URLs within https://github.com/sagemath/sage-wheels/releases

Depends on #31396

Component: distribution

Branch: u/mkoeppe/modularized_relocatable_wheel_sage_conf

Issue created by migration from https://trac.sagemath.org/ticket/31602