integrate the SAT-based resolver into Pkg

[KristofferC]

This makes the SAT resolver (https://github.com/StefanKarpinski/Resolver.jl) a new default (so that people actually try it). We still need to make libpicosat available from julia itself (JuliaLang/julia#59119).

This adds an extra CI run that runs with the maxsum resolver by default so that we properly test that.

[DilumAluthge]

Should we make this "pluggable"? E.g., instead of special-casing StefanKarpinski/Resolver.jl, could we have a way for the user to provide a Resolver struct, and then Pkg uses that? Obviously we'd need to write out the interface spec for Resolver objects. But that would allow anyone to experiment with their own Resolver and then drop it in?

[KristofferC]

I think that is kind of a premature generalisation.