Change PackageFixedDeps to use installed IDs

Take advantage in cabal-install of the new
HasInstalledPackageId/PackagedInstall split in Cabal.  The graph traversal
functions in cabal-install, previously redundant, are now back in use. Their
types match the ones in Cabal, with only the difference in the PackageInstalled
(Cabal) versus PackageFixedDeps (cabal-install) type class.

The only PackageInstalled instance left in Cabal is for InstalledPackage, which
is a thin wrapper around InstalledPackageInfo; with these refactorings in
place, InstalledPackage is there only to support the TopDown solver. The fact
that we won't have PackageInstalled instances anymore for PlanPackage and co
means that we are forced to call the correct graph traversal functions (from
cabal-install, rather than from Cabal).