gh-37857: Replace special handling of optional extensions (bliss, coxeter3, ....)
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We relieve the distribution **sagemath-standard** (in both versions -
`SAGE_ROOT/pkgs/sagemath-standard` and `SAGE_ROOT/src`) from the duty to
build "optional extensions" based on what Sage packages are installed.
The installation is now done uniformly using the modularized
distributions **sagemath-bliss**, **sagemath-coxeter3** etc.
We introduce the missing features `coxeter3` and `sirocco` so that the
doctester does not have to rely on the sage-the-distro installation
records any more.
The wheels of the distributions now build correctly even when not going
through building an sdist first, which previously was required to apply
MANIFEST-based filtering. This is achieved using the new
`sage_setup.command.build_py`.
User-visible change:
- To install these options, use `./configure --enable-sagemath_bliss`
before building, or use `./sage -i sagemath_bliss` or `make
sagemath_bliss`.
### :memo: Checklist
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preview.
### :hourglass: Dependencies
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- Depends on #37737 (merged here)
- Depends on #37973 (merged here)
URL: https://github.com/sagemath/sage/pull/37857
Reported by: Matthias Köppe
Reviewer(s): François Bissey
