[GHC] #10566: Move "package key" generation to GHC

[GHC]

#10566: Move "package key" generation to GHC
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        Reporter:  ezyang            |                   Owner:  ezyang
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Package system    |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:  Phab:D1056
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Comment (by ezyang):

 I was thinking about this situation some more, and I realized that we can
 actually avoid having to have Cabal query GHC for the package key, if we
 adopt a convention: the version hash of a package is the SAME as the
 package key, if the package is definite. I've repushed the pull request
 with this in mind.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10566#comment:9>
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