Stop wrapping Julia modules in special objects

[fingolfin]

This resolves issues with outdated caches in our module wrappers.

We added these wrappers to serve as a cache. But we already exempted Julia functions from this. For which we create wrapper objects -- so arguably the cache would make most sense for them... But this can cause inconsistencies, hence @ThomasBreuer remove this in PR #423 four years ago.

So the only thing we cached were the wrapper objects for Julia modules. And the only reasons we needed those wrappers were ...

1. to be able to do caching (haha),
2. to be able to supply the "fake" Julia.GAP as a way to access the GAP Julia module,
3. for some printing tweaks.

This PR achieves all of the above by adjusting the GAP type for Julia object slightly, we now indicate whether we are seeing a Julia module by setting a suitable filter. As an added bonus, this now also works with modules access by any other means, e.g. returned by a JuliaEvalString or a Julia function.

The part I like least is the new hack to make Julia.GAP possible. A long-term alternative to this might be to provide a different clean way to access this -- e.g. perhaps we should provide a global GAP variable GAP_jl which contains the GAP module? (We could also just call it GAP but I feel this might be reeeaaally confusing to GAP users, so I'd rather not). For symmetry in Oscar we could also add Oscar_jl as an alias for the Oscar global.

But all of that is beyond this PR. We could file it as a separate issue if care enough.

Resolves #1044
Closes #1045

[fingolfin]

(This should only be merged after #1029, as getting that one right is much harder)

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 2 lines in your changes missing coverage. Please review.

Project coverage is 75.58%. Comparing base (e60382d) to head (5a8d7e0).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/JuliaInterface/gap/JuliaInterface.gi	90.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1049      +/-   ##
==========================================
- Coverage   75.61%   75.58%   -0.03%     
==========================================
  Files          55       55              
  Lines        4478     4477       -1     
==========================================
- Hits         3386     3384       -2     
- Misses       1092     1093       +1     

Files with missing lines	Coverage Δ
pkg/JuliaInterface/gap/JuliaInterface.gd	100.00% <100.00%> (ø)
pkg/JuliaInterface/read.g	100.00% <ø> (ø)
pkg/JuliaInterface/src/JuliaInterface.c	74.81% <100.00%> (+0.57%)	⬆️
pkg/JuliaInterface/gap/JuliaInterface.gi	91.86% <90.00%> (-1.40%)	⬇️

[lgoettgens]

I don't feel qualified to comment on the changed internals here. Anyway, could you add a test for #1044, e.g. the one from https://github.com/oscar-system/GAP.jl/pull/1045/files#diff-f673fbc29b03e12764445c880278a94061eb0b24269b0fd2621c27c604d50b1e?

[fingolfin]

@lgoettgens done, I shamelessly stole it from your PR :-)

[lgoettgens]

@lgoettgens done, I shamelessly stole it from your PR :-)

I stole it from your issue, so I think we're even now :)