Fix opam admin check --obsolete

[kit-ty-kate]

As noted in #5748, opam admin check --obsolete currently doesn’t work for its intended purpose.

Instead of ignoring available and filters (I haven’t looked at the code, I’m just guessing based on behaviour) it should probably:

• for each packages
  • list all variables they’re using
    • for each uses call the solver with the variable set to the possible values (the tricky part)
    • add all those possibilities to the result

This should also probably get rid of the need to have a --ignore-test-doc argument.
This could also fix #4541 since it’s sharing some code

[kit-ty-kate]

cc @Armael @gasche since you both worked on something similar in the past to try and fix #4541.
Sorry the pseudo algorithm is very handwavy, this is very much a back of a napkin type thing and I’m not sure if the third point is even possible in particular, i think it’s probably too tricky to do precisely but an heuristic when you just evaluate all expression to true and false then solve N-time seems fine. Do you have any insights by any chance?