Introduce SAGE_OPTIONAL_PACKAGES_ASSUME_INSTALLED env variable

[antonio-rojas]

Currently, the code in pkgs/sagemath-standard/setup.py that decides whether or not to build optional extensions relies on the is_package_installed_and_updated function, which is specific to sage-the-distro. Therefore, downstream distros need to patch this if they want to build optional extensions.

This PR introduces a SAGE_OPTIONAL_PACKAGES_ASSUME_INSTALLED env variable that distros can use to specify which optional extensions to build, regardless of the is_package_installed_and_updated output.

Eventually this code should be ported away from is_package_installed_and_updated and use pkgconfig to detect optional libraries instead, but in the meantime this will save distros some patching.

[mkoeppe]

This code in pkgs/sagemath-standard/setup.py is only there for legacy monolithic installs (./configure --disable-editable --disable-wheels), and intended to be removed soon:

• See Use src/setup.py for the installation mode --disable-editable --disable-wheels #34633

I'd strongly recommend that downstream use the new distributions sagemath-bliss etc. added in #35661 to build the optional packages.

• See pkgs/sagemath-meataxe/README.rst: Remove markup unknown to PyPI #36003

[github-actions]

Documentation preview for this PR (built with commit bd096a2; changes) is ready! 🎉

[kiwifb]

It is neater than my own work around. But I pretty much drank the split package kool aid. I am quite happy that #35661 is merged because now, I will not have to treat develop and volker's branch differently from what I do the release. The downside is that I have more package to maintain.

I do not feel particularly inclined to review this positively given that the whole thing is finally going and I will not miss it. Splitting those optional things is much better than the half backed detection historical code. The fact that we had to tiptoe around sage packaging system and that now we do not need to anymore is welcome news to me.

[antonio-rojas]

Alright, I'm happy as long as I can keep building everything from github.