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

[GHC]

#10566: Move "package key" generation to GHC
-------------------------------------+-------------------------------------
        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:
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Comment (by ezyang):

 Another reason it's important to support `-package-name` separately from
 `-version-hash` is that they can vary separately.  Consider a Backpack
 file that looks like:

 {{{
     package p where
        ...

     package q where
        ...
 }}}

 both of which come from the same Hackage distribution unit `p` (i.e. you
 can `cabal install p`).  Since GHC is responsible for building both p and
 q at the same time (and Cabal knows nothing about these subpackages), the
 version hash of these packages has to be the same. However, we still have
 to distinguish package p and package q, so here we vary the package name.
 So it has semantic meaning too!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10566#comment:6>
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