Refactor solver flags to have common prefix

Especially if we're adding more stuff like https://github.com/haskell/cabal/pull/8261/ it would be good to also do a refactor to create a single datastructure representing all solver flags (and putting that datastructure in the solver library itself). We could also consolidate solver flag parsing, and move to a convention where all flags come with a `--solver-` prefix (with aliases for now to preserve existing flags). This should let us extend with more solver flags without the whole thing becoming too confusing and ad-hoc, and help with discoverability since the flags would all be listed together.