In this commit we modify the _output_ of the modular solver (CP, the modular's
solver internal version of ConfiguredPackage) to have fine-grained dependency.
This doesn't yet modify the rest of cabal-install, so once we translate from CP
to ConfiguredPackage we still lose the distinctions between different kinds of
dependencies; this will be the topic of the next commit.

In the modular solver (and elsewhere) we use Data.Graph to represent the
dependency graph (and the reverse dependency graph). However, now that we have
more fine-grained dependencies, we really want an _edge-labeled_ graph, which
unfortunately it not available in the `containers` package. Therefore I've
written a very simple wrapper around Data.Graph that supports edge labels; we
don't need many fancy graph algorithms, and can still use Data.Graph on these
edged graphs when we want (by calling them on the underlying unlabeled graph),
so adding a dependency on `fgl` does not seem worth it.