[GHC] #10714: After implementing new installed package ID (hash of sdist), get rid of package keys

[GHC]

#10714: After implementing new installed package ID (hash of sdist), get rid of
package keys
-------------------------------------+-------------------------------------
              Reporter:  ezyang      |             Owner:  ezyang
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Package     |           Version:  7.10.2
  system                             |
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
-------------------------------------+-------------------------------------
 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.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10714>
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