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.
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.

sdiehl/ghc,acowley/ghc,ekmett/ghc,mcmaniac/ghc,da-x/ghc,tjakway/ghcjvm,mfine/ghc,nomeata/ghc,gridaphobe/ghc,gridaphobe/ghc,acowley/ghc,jstolarek/ghc,nushio3/ghc,urbanslug/ghc,forked-upstream-packages-for-ghcjs/ghc,olsner/ghc,nathyong/microghc-ghc,christiaanb/ghc,frantisekfarka/ghc-dsi,ezyang/ghc,tjakway/ghcjvm,GaloisInc/halvm-ghc,holzensp/ghc,AlexanderPankiv/ghc,ezyang/ghc,ml9951/ghc,da-x/ghc,urbanslug/ghc,AlexanderPankiv/ghc,gcampax/ghc,holzensp/ghc,holzensp/ghc,nkaretnikov/ghc,oldmanmike/ghc,tibbe/ghc,mfine/ghc,elieux/ghc,shlevy/ghc,mcmaniac/ghc,vTurbine/ghc,tjakway/ghcjvm,ghc-android/ghc,snoyberg/ghc,oldmanmike/ghc,nomeata/ghc,elieux/ghc,snoyberg/ghc,lukexi/ghc-7.8-arm64,sgillespie/ghc,christiaanb/ghc,ghc-android/ghc,ekmett/ghc,mettekou/ghc,TomMD/ghc,green-haskell/ghc,nathyong/microghc-ghc,jstolarek/ghc,tjakway/ghcjvm,christiaanb/ghc,sdiehl/ghc,ryantm/ghc,shlevy/ghc,urbanslug/ghc,spacekitteh/smcghc,shlevy/ghc,sdiehl/ghc,TomMD/ghc,anton-dessiatov/ghc,lukexi/ghc,christiaanb/ghc,holzensp/ghc,bitemyapp/ghc,sgillespie/ghc,hferreiro/replay,da-x/ghc,frantisekfarka/ghc-dsi,wxwxwwxxx/ghc,snoyberg/ghc,AlexanderPankiv/ghc,nathyong/microghc-ghc,green-haskell/ghc,da-x/ghc,bitemyapp/ghc,nomeata/ghc,vikraman/ghc,spacekitteh/smcghc,nkaretnikov/ghc,ml9951/ghc,mcmaniac/ghc,gridaphobe/ghc,ghc-android/ghc,elieux/ghc,ghc-android/ghc,nathyong/microghc-ghc,urbanslug/ghc,nushio3/ghc,bitemyapp/ghc,nkaretnikov/ghc,ml9951/ghc,vikraman/ghc,gridaphobe/ghc,bitemyapp/ghc,shlevy/ghc,nushio3/ghc,siddhanathan/ghc,fmthoma/ghc,snoyberg/ghc,urbanslug/ghc,mettekou/ghc,fmthoma/ghc,fmthoma/ghc,vTurbine/ghc,lukexi/ghc-7.8-arm64,hferreiro/replay,ryantm/ghc,fmthoma/ghc,olsner/ghc,green-haskell/ghc,vikraman/ghc,TomMD/ghc,mcschroeder/ghc,gcampax/ghc,lukexi/ghc,tjakway/ghcjvm,nushio3/ghc,nushio3/ghc,sdiehl/ghc,lukexi/ghc-7.8-arm64,mcschroeder/ghc,christiaanb/ghc,TomMD/ghc,nathyong/microghc-ghc,ryantm/ghc,AlexanderPankiv/ghc,elieux/ghc,siddhanathan/ghc,oldmanmike/ghc,vTurbine/ghc,gcampax/ghc,nathyong/microghc-ghc,nkaretnikov/ghc,sgillespie/ghc,mfine/ghc,forked-upstream-packages-for-ghcjs/ghc,ml9951/ghc,mcschroeder/ghc,tibbe/ghc,fmthoma/ghc,lukexi/ghc,fmthoma/ghc,mfine/ghc,GaloisInc/halvm-ghc,wxwxwwxxx/ghc,wxwxwwxxx/ghc,snoyberg/ghc,lukexi/ghc,ilyasergey/GHC-XAppFix,elieux/ghc,acowley/ghc,olsner/ghc,elieux/ghc,ml9951/ghc,vikraman/ghc,forked-upstream-packages-for-ghcjs/ghc,anton-dessiatov/ghc,acowley/ghc,ryantm/ghc,mettekou/ghc,TomMD/ghc,tibbe/ghc,hferreiro/replay,tjakway/ghcjvm,vikraman/ghc,ghc-android/ghc,TomMD/ghc,da-x/ghc,ezyang/ghc,ml9951/ghc,jstolarek/ghc,mcmaniac/ghc,vTurbine/ghc,GaloisInc/halvm-ghc,shlevy/ghc,mcschroeder/ghc,jstolarek/ghc,acowley/ghc,nomeata/ghc,nomeata/ghc,forked-upstream-packages-for-ghcjs/ghc,hferreiro/replay,gridaphobe/ghc,nushio3/ghc,sgillespie/ghc,hferreiro/replay,ezyang/ghc,ghc-android/ghc,bitemyapp/ghc,ezyang/ghc,spacekitteh/smcghc,acowley/ghc,sgillespie/ghc,olsner/ghc,sdiehl/ghc,forked-upstream-packages-for-ghcjs/ghc,anton-dessiatov/ghc,anton-dessiatov/ghc,gcampax/ghc,tjakway/ghcjvm,ghc-android/ghc,anton-dessiatov/ghc,spacekitteh/smcghc,wxwxwwxxx/ghc,mcschroeder/ghc,mcschroeder/ghc,GaloisInc/halvm-ghc,olsner/ghc,sgillespie/ghc,gridaphobe/ghc,wxwxwwxxx/ghc,ekmett/ghc,gridaphobe/ghc,jstolarek/ghc,gcampax/ghc,ilyasergey/GHC-XAppFix,forked-upstream-packages-for-ghcjs/ghc,snoyberg/ghc,GaloisInc/halvm-ghc,frantisekfarka/ghc-dsi,siddhanathan/ghc,mettekou/ghc,anton-dessiatov/ghc,nkaretnikov/ghc,elieux/ghc,nathyong/microghc-ghc,hferreiro/replay,TomMD/ghc,holzensp/ghc,mfine/ghc,ilyasergey/GHC-XAppFix,spacekitteh/smcghc,tibbe/ghc,oldmanmike/ghc,nkaretnikov/ghc,olsner/ghc,gcampax/ghc,nushio3/ghc,shlevy/ghc,vikraman/ghc,nkaretnikov/ghc,sgillespie/ghc,AlexanderPankiv/ghc,green-haskell/ghc,christiaanb/ghc,ilyasergey/GHC-XAppFix,vikraman/ghc,ml9951/ghc,vTurbine/ghc,sdiehl/ghc,mfine/ghc,mcschroeder/ghc,ml9951/ghc,GaloisInc/halvm-ghc,wxwxwwxxx/ghc,vTurbine/ghc,frantisekfarka/ghc-dsi,vTurbine/ghc,sdiehl/ghc,shlevy/ghc,mettekou/ghc,anton-dessiatov/ghc,ekmett/ghc,urbanslug/ghc,ezyang/ghc,christiaanb/ghc,GaloisInc/halvm-ghc,lukexi/ghc,mcmaniac/ghc,fmthoma/ghc,siddhanathan/ghc,siddhanathan/ghc,mfine/ghc,mettekou/ghc,olsner/ghc,lukexi/ghc-7.8-arm64,siddhanathan/ghc,urbanslug/ghc,frantisekfarka/ghc-dsi,oldmanmike/ghc,forked-upstream-packages-for-ghcjs/ghc,mettekou/ghc,da-x/ghc,tibbe/ghc,ezyang/ghc,green-haskell/ghc,lukexi/ghc-7.8-arm64,snoyberg/ghc,AlexanderPankiv/ghc,da-x/ghc,acowley/ghc,oldmanmike/ghc,oldmanmike/ghc,AlexanderPankiv/ghc,gcampax/ghc,ryantm/ghc,hferreiro/replay,siddhanathan/ghc,ekmett/ghc,wxwxwwxxx/ghc