Change ProductOfSets to remove dependence on implementation in MOI

[odow]

I wanted to update MOI (jump-dev/MathOptInterface.jl#3022), but this part of DiffOpt depends on the internal fields.

The change to MOI is theoretically breaking (as evidenced by the need for this PR), so that is a draft for discussion. We might never merge the change in MOI, but this change could be merged regardless.

[codecov]

Codecov Report

❌ Patch coverage is 79.26829% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.46%. Comparing base (f3fdbd8) to head (780b85c).

Files with missing lines	Patch %	Lines
src/product_of_sets.jl	78.75%	17 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #372      +/-   ##
==========================================
- Coverage   91.88%   91.46%   -0.42%     
==========================================
  Files          17       17              
  Lines        2722     2788      +66     
==========================================
+ Hits         2501     2550      +49     
- Misses        221      238      +17     

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[blegat]

Can the ProductOfSets in DiffOpt be simplified to be able to reuse against the implementation of the MOI one ? Actually, I always wanted to move this ProductOfSets of DiffOpt to MOI, it is also useful for MatrixOfConstraints and would be for any MOI model that can support arbitrary sets. Maybe we can move it to MOI first and then do jump-dev/MathOptInterface.jl#3022