Relax dependencyInconsistencies to allow the base-3,4 thing
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.
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