... including for packages that we cannot use (yet)

This is for several purposes:

• Test proper isolation of our build (i.e., the presence of unused system packages do not break our build) using tox -e ...-maximal
• Testing sagelib without Sage-the-distribution (similar to Add SAGE_ROOT/environment.yml for "conda env create -f". #28745/Fixes for the conda-for-Sage-developers installation method, add GH Actions workflow #30845 for conda)
• Preparation for tickets that will add spkg-configure.m4

(from #30371)

Depends on #29630

CC: @tobiasdiez @slel @dimpase

Component: build: configure

Author: Tobias Diez

Branch/Commit: cb0fc85

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/30859