Rework of the JuliaPackage easyblock

[Crivella]

Summary of changes

• Adds compile_cache check to sanity checks to ensure that the compiled version of the package can actually be found/re-used
• Allow specifying test-only dependencies with the is_test_dependency EC param
• Allow turning on Julia's verbose output with the julia_debug EC param
• Allow installing subpackages of a package (eg optional dependencies) by specifying their root dir relative to the toplevel package with the subpackages_dirs EC param
• Allow reusing packages from a dependency (fix JuliaPackage can't really work with depedencies #4123)
  • The environments of the dependencies are merged into the one of the package being installed
• Use Pkg.instantiate() to install all extensions instead of installing them one by one with Pkg.develop
• Remove need for deps to be ordered in the EC files
• Makes use of Julia's capability to install extension in parallel (still respecting the max_parallel option)
• Allows running package tests in a dedicated step
  • Can work in offline mode but many packages might require online to work properly due to how the temporary environment are build by some test runner themselves
• Switch to using an EB specific environment variable to keep track of the LOAD/DEPOT PATHS and use them to correctly set them for Julia in a site-specific startup script

Maybe TODO

• Improve Lua/Tcl syntax in module footer
• Modify Julia itself similar to https://github.com/easybuilders/JSC/blob/6285a066b6debed28211730afb8ffc5eb2430c2f/Custom_EasyBlocks/julia.py to allow the interpreter to pick up the DEPOT/LOAD path directly from the new env variables
• [ ]Test how reusable deps really are. The original approach aimed at copying all packages from the deps into the current package and re-install everything. Here we actually re-use them but the results seems to be dependent on the load order (see discussion in JuliaPackage can't really work with depedencies #4123) so if the dep order is switched w.r.t. the one at build time slow re-compilation might happen

Notes

This PRs includes the changes from:

• Fix for running --sanity-check-only for a JuliaBundle #4122
• JuliaPackage: Allow running tests in a dedicated test_step #4124

Fixes:

• JuliaPackage can't really work with depedencies #4123

[Crivella]

Moved the final setting of LOAD_PATH and DEPOT_PATH to Julia's site-specific startup script.
This resolves:

• having to write custom module code
• the reversing setenv problem
• Lua-vs-Tcl syntax

[Flamefire]

This looks odd. How does this work? It (may) designate a whole easyconfig as test-only making the files missing from the installed module.

We should avoid:
a) That it is set on anything but a (bundle-)extension
b) That it turns up in extensions of the module.

[Crivella]

Yeah should probably add more checks around that.
Right now i am considering what i want to do with it, and if i should shift this from a flag to single extensions to just treating build-deps as test-only dependencies.

The main thing that i am focusing on now is how feasible it is is re-using deps. It is working but is fairly sensitive. If 2 ECs ships the same Julia package, it is possible that the compile cache will be broken based on load order.
The check i've added right now only works if the 2 ECs are being used as a deps of another.
The other point is that if we go this route every Bundle should ideally define only 1 exts (which begs the question if bundles are needed at all), otherwise we go back to the old intended (but broken way) of just using deps as a fancy way to tell Julia which packages to re-install but not really use the deps themselves (still think that solution might not really allow to load multiple packages at the same time)