[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
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        Reporter:  ezyang            |                   Owner:  ezyang
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Package system    |                 Version:  7.10.2
      Resolution:                    |                Keywords:  backpack
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by ezyang):

 Another point: when Backpack is back in the picture, `UnitKey` only equals
 `InstalledPackageId` if the package in question is a non-Backpack package
 (installed package ID is per package, but a package can have multiple
 units.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10714#comment:3>
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