This part adds the InstalledPackageIndex type to
Distribution.Simple.PackageIndex.  Now that packages have a unique
identifier within a package database, it makes sense to use this as
the key for looking up installed packages, so InstalledPackageIndex is
a mapping from InstalledPackageId to InstalledPackageInfo.

Distribution.Simple.PackageIndex still supports other kinds of package
mappings: PackageIndex is a mapping from PackageName.

All the functions in the section "Special Queries" now work on
InstalledPackageIndex rather than PackageFixedDeps pkg => PackageIndex
pkg:

  topologicalOrder,
  reverseTopologicalOrder,
  dependencyInconsistencies,
  dependencyCycles,
  brokenPackages,
  dependencyClosure,
  reverseDependencyClosure
  dependencyGraph