[breaking] refactor linear solver loading to use Pkg extensions

[odow]

This is a WIP PR to flesh out what a refactored SCS.jl would look like if we used package extensions.

CI will be broken until at least

• [SCS] update to v3.2.4 JuliaPackaging/Yggdrasil#7671
• [SCS_MKL] update to v3.2.4 JuliaPackaging/Yggdrasil#7672
• [SCS_GPU] update to v3.2.4 JuliaPackaging/Yggdrasil#7673

[kalmarek]

@odow looks rather good, no more requires, init etc, much cleaner;

I'd suggest however to wait with this until the a new LTS is released which does support extensions. In some sense the current situation, while not optimal just works, and I don't feel like putting more burden of maintaining two incompatible versions of SCS.jl

[codecov]

Codecov Report

Attention: 17 lines in your changes are missing coverage. Please review.

Comparison is base (79917cc) 90.35% compared to head (24824ad) 87.87%.

Files	Patch %	Lines
ext/SCSSCS_GPU_jllExt.jl	0.00%	14 Missing ⚠️
ext/SCSSCS_MKL_jllExt.jl	85.71%	2 Missing ⚠️
src/SCS.jl	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #286      +/-   ##
==========================================
- Coverage   90.35%   87.87%   -2.48%     
==========================================
  Files           8        8              
  Lines         425      429       +4     
==========================================
- Hits          384      377       -7     
- Misses         41       52      +11     

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[odow]

@kalmarek I'm trying to sort it so that we support @requires on 1.6 and the package extensions on 1.9.

We'll still have to do a breaking change to the use package loading, but that's okay. When 1.9 becomes the LTS, we can drop the requires part.

[kalmarek]

Looks great!

[odow]

cc @blegat you should take a look at this. Nominally it's a breaking change, but it affects only the lazy loading of the MKL and GPU solvers.

[blegat]

Great work, it's much cleaner now