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

[sebright]

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, including resolveDependencies and DepResolverParams:

cabal/cabal-install/src/Distribution/Client/Dependency.hs

Lines 779 to 790 in 91769ab

	-- | Run the dependency solver.
	--
	-- Since this is potentially an expensive operation, the result is wrapped in a
	-- a 'Progress' structure that can be unfolded to provide progress information,
	-- logging messages and the final result or an error.
	resolveDependencies
	:: Platform
	-> CompilerInfo
	-> Maybe PkgConfigDb
	-> DepResolverParams
	-> Progress String String SolverInstallPlan
	resolveDependencies platform comp pkgConfigDB params =

cabal/cabal-install/src/Distribution/Client/Dependency.hs

Lines 173 to 176 in 91769ab

	-- | The set of parameters to the dependency resolver. These parameters are
	-- relatively low level but many kinds of high level policies can be
	-- implemented in terms of adjustments to the parameters.
	data DepResolverParams = DepResolverParams