Fix precompilation issue

[devmotion]

Fixes #1087 (basically in the same way as the implementation in NLOpt, the tiny difference is just that the check in the __init__ function in the extension is also based on isdefined(Base, :get_extension) to ensure there are no cases when Optim.Optimizer is not defined).

This reintroduces MOI as a regular dependency on Julia < 1.9, but IMO this is fine since on Optim <= 1.9.2 MOI was a regular dependency already. On Julia >= 1.9, this PR actually reduces the number of dependencies since PackageExtensionsCompat is removed.

[pkofod]

thanks David!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.46%. Comparing base (7cc8328) to head (1961975).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1088      +/-   ##
==========================================
+ Coverage   85.43%   85.46%   +0.02%     
==========================================
  Files          45       45              
  Lines        3276     3274       -2     
==========================================
- Hits         2799     2798       -1     
+ Misses        477      476       -1     

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