Give the top-down solver it's own copy of `dependencyGraph`. This means that we
now have three independent implementations of `dependencyGraph`:

- `dependencyGraph` in `Cabal` takes a package index indexed by installed
  package IDs and only has access to library dependencies.
- `dependencyGraph` in `Distribution.Client.PlanIndex` in `cabal-install` takes
  a package index indexed by installed package IDs and has access to all
  dependencies.
- `dependencyGraph` in the top-down solver in `cabal-install` takes a package
  index indexed by package _names_, and has access to all dependencies.

Ideally we would switch the top-down solver over to use a package indexed by
installed package IDs, so that this duplication could be avoided, but that's a
bit of work and the top-down solver is legacy code anyway. Can still do that
later, of course.

Moreover, this makes the top-down solver monomorphic where possible, and
introduce its own SourceDeps class so that it is independent of the FixedDeps
class (which we will change over to use InstalledPackageIds instead).