./bootstrap && ./sage -sh -c '(cd pkgs/sagemath-standard && tox -v -v -v -e sagepython-sagewheels-nopypi-norequirements)' fails in 9.8.beta5 because the sdist is broken

  cimport cython
  from sage.algebras.fusion_rings.poly_tup_engine cimport (
  ^
  ------------------------------------------------------------

  sage/algebras/fusion_rings/fast_parallel_fmats_methods.pyx:12:0: 'sage/algebras/fusion_rings/poly_tup_engine.pxd' not found

We fix this by

• updating MANIFEST
• unconditionally running the module finder in pkgs/sagemath-standard/setup.py

We make the same change also in pkgs/sagemath-objects/setup.py (by symlink, this also affects sagemath-categories).

Depends on #34859

CC: @kwankyu @kiwifb

Component: distribution

Author: Matthias Koeppe

Branch/Commit: e229436

Reviewer: François Bissey

Issue created by migration from https://trac.sagemath.org/ticket/34858