Break up Distribution.Package -- only classes and reexports remain

Cabal/Cabal.cabal

Cabal/Distribution/Backpack.hs

Cabal/Distribution/Backpack/ConfiguredComponent.hs

Cabal/Distribution/Backpack/DescribeUnitId.hs

Cabal/Distribution/Backpack/FullUnitId.hs

Cabal/Distribution/Backpack/Id.hs

Cabal/Distribution/Backpack/LinkedComponent.hs

Cabal/Distribution/Backpack/MixLink.hs

Cabal/Distribution/Backpack/ModuleScope.hs

Cabal/Distribution/Backpack/PreExistingComponent.hs

Cabal/Distribution/Backpack/ReadyComponent.hs

Cabal/Distribution/Package.hs

Cabal/Distribution/PackageDescription/Check.hs

Cabal/Distribution/PackageDescription/Configuration.hs

Cabal/Distribution/PackageDescription/Parse.hs

Cabal/Distribution/Parsec/Class.hs

Cabal/Distribution/Simple/Build/Macros.hs

Cabal/Distribution/Simple/GHC/IPI642.hs

Cabal/Distribution/Simple/GHC/IPIConvert.hs

Cabal/Distribution/Simple/GHC/Internal.hs