Previously we said a package graph was inconsistent if two
dependencies on the same package name specified different
versions. Now we say that two such dependencies on different
versions are ok if there is a direct dependency between those
two package versions. So if your package graph ends up with
both base 3 and base 4 in it, then that's ok because base 3
directly depends on base 4, so we declare it not to be an
inconsistency. This removes the scary warnings at configure
time (fixing ticket #366) and also adjusts the invariant and
assertion of the InstallPlan ADT in cabal-install.