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.

Trac metadata

Trac field	Value
Version	7.10.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Package system
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