[GHC] #10622: Rename Backpack packages to units

[GHC]

#10622: Rename Backpack packages to units
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        Reporter:  ezyang            |                   Owner:  ezyang
            Type:  task              |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.11
      Resolution:                    |                Keywords:  backpack
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:  Phab:D1057
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Comment (by simonpj):

 > Installed units continue to be identified by `InstalledPackageId`

 Wouldn't `InstalledUnitId` be clearer?  Indeed wouldn't
 `InstalledPackageId` be positively misleading?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10622#comment:10>
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