Fix bug when the combined package DB has duplicate package IDs

Someone on IRC reported that they were encountering a strange missing
module error from GHC, and it turned out that they had mtl-1.1.0.2
installed in both the global and local DB, with the same
InstalledPackageId.  The two packages should be interchangeable, but
we were forgetting to eliminate duplicates in part of the
package-processing phase in GHC.

Really I should switch this code over to use the new PackageIndex data
type in Cabal, which ought to make things simpler.

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