Make sure that code in src/ does not use code in experimental/

[lgoettgens]

This is stated in a big red box in https://docs.oscar-system.org/dev/Experimental/intro/#Purpose but not enforced.

For stability guarantees of Oscar 1.x related to the experimental folder (see #2011), we should try to enforce the above rule, or at least create a list of issues and decide to not deal with them for now.

Potential roadmap:

• Make experimental packages more self-contained #3409: Adapt loading of experimental packages to be more self-contained. This makes it then possible to delete a module in experimental without the need to adapt anything outside of that.
• Add CI job that runs tests without the experimental folder #3525: Add a CI job that deletes everything in the experimental folder (but a whitelist) and tries to run the Oscar tests. In the beginning, we would put most of the current experimental folders into the whitelist. A similar job should check if the documentation can still be build without experimental projects (aka experimental's docstrings are not contained in non-experimental doc pages).
• Long-term: Try to remove things from the whitelist. Adapt them or the calling code to keep everything working. Eventually, the whitelist should be empty.
  • DoubleAndHyperComplexes: MethodError: no method matching simplify(::SubquoModule{MPolyQuoRingElem{MPolyDecRingElem{FqMPolyRingElem, AbstractAlgebra.Generic.MPoly{FqMPolyRingElem}}}})
  • ExteriorAlgebra undefined binding 'exterior_algebra' in `@docs` block in src/NoncommutativeAlgebra/PBWAlgebras/quotients.md:40-42 and Error During Test at /home/runner/work/Oscar.jl/Oscar.jl/test/Rings/PBWAlgebraQuo.jl:41
  • GaloisGrp: no docs found for 'fixed_field(C::Oscar.GaloisGrp.GaloisCtx, s::Vector{PermGroup})' in `@docs` block in src/NumberTheory/galois.md:275-278
  • GModule: many doctest failures and MethodError: no method matching numerator(::QQPolyRingElem)
  • InvariantTheory: undefined binding 'linearly_reductive_group' in `@docs` block in src/InvariantTheory/reductive_groups.md:53-55` and more docs errors
  • ModStd: MethodError: no method matching monomial(::QQMPolyRing, ::Vector{Int64}) and many similar errors
  • Schemes

@thofma and @joschmitt liked the idea.