Removing pip's legacy resolver

Hi folks!

If you are aware of any limitation of the new stricter resolver (other than the fact that it’s strict and doesn’t allow “relaxation” of that strictness), please flag that in this thread. There’s one known blocker, around the interaction of extras and hashes. If there’s anything else, please say so here or on the aforementioned thread.

I’d like to start making progress on the legacy resolver removal, since it’s blocking a couple of meaningful major changes to the codebase, including paying down some of our technical debt.