Move remaining dependency solver API and unit tests into cabal-install-solver package

This issue is a followup to #3781. The main dependency solver modules were moved into a separate package (`cabal-install-solver`), but some functions that act as an API are still in `cabal-install`. The dependency solver unit tests use that API, so they are also in `cabal-install`.

This change is much simpler now that the top-down solver was removed in #3598 and support for multiple solvers was removed in #9282.

Most of the remaining dependency solver API is in [Distribution.Client.Dependency](https://github.com/haskell/cabal/blob/master/cabal-install/src/Distribution/Client/Dependency.hs), including `resolveDependencies` and `DepResolverParams`:

https://github.com/haskell/cabal/blob/91769ab183f467f352c786c73f566e09ebb73aa7/cabal-install/src/Distribution/Client/Dependency.hs#L779-L790

https://github.com/haskell/cabal/blob/91769ab183f467f352c786c73f566e09ebb73aa7/cabal-install/src/Distribution/Client/Dependency.hs#L173-L176