Solver: Prefer flag and stanza goals that have only one choice

I discussed this with @kosmikus. If a flag or stanza has only one choice, e.g, the user has enabled tests, the solver should assign a value as soon as possible, so that it can discover new dependencies more quickly. The behavior would be similar to `--reorder-goals`, but less general. It would also avoid the slowdown in tree traversal that is caused by `--reorder-goals`. Another possibility is to also prefer package goals where the user has specified an exact version.

This change would handle the issue with stanza preferences discussed here: https://github.com/haskell/cabal/issues/3453#issuecomment-222580111