levels of incompatibility

[StefanKarpinski]

Problem: We've had a recent situation where JuliaPro ships with a fixed set of package versions which were compatible at the time it was released, but a subsequent update to METADATA (JuliaLang/METADATA.jl#12106, see discussion in JuliaLang/METADATA.jl#12264) caused that set of versions to become incompatible. Unfortunately, this means that doing almost anything with that set of packages causes the resolver to say that the operation is not allowed because it leads to an incompatible ("infeasible") set of package versions.

Potential solution: In general, it would be good to allow operations that don't make the current situation any worse, and in general to afford users the option to pick their poison in situations where no fully compatible resolution exists. This seems to lead to a notion of "levels of incompatibility" for sets of versions rather than the current black-and-white notion of compatible or not. If you're already at a level of incompatibility, then an operation which doesn't make that level worse should be allowed without warning. If there are no solutions which don't make the situation worse, then it would be ideal to prompt the user with the best options for resolving the situation as well as possible.