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BC note: renamed type PackageIndex :: * to InstalledPackageIndex.

The intent is to have cabal-install use this package index in order to
track information about compilation in progress.  We introduce a new
PackageInstalled type-class to keep track of data types which have their
IDs and dependency graphs in InstalledPackageId.
Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>