After implementing new installed package ID (hash of sdist), get rid of package keys
GHC tracking bug for https://github.com/haskell/cabal/issues/2745
Externally, GHC's flags do not have to change much; a user simply passes the installed package ID to the flag currently named
-this-package-key (but perhaps we should rename this.)
Internally, if we can assume that
PackageKey == InstalledPackageId, we can do away with the
InstalledPackageId map and get rid of the level of indirection between the bin-pkg-db (which records installed package IDs`) and GHC's guts (which record package keys).
Blocked on Cabal not actually using ABI hashes to identify packages.