(split out from #31396)

We refactor sage_conf to make it easier to maintain variants of it, both the 2 in-tree variants (and another one to be added in #31396) and downstream variants:'''

• The configure-generated configuration file is now pkgs/sage-conf/_sage_conf/_conf.py
• The file pkgs/sage-conf/sage_conf.py is now static

CC: @kiwifb @antonio-rojas @tornaria

Component: scripts

Author: Matthias Koeppe

Branch/Commit: eeddb03

Reviewer: François Bissey

Issue created by migration from https://trac.sagemath.org/ticket/33295