[GHC] #11015: Add note describing unit ids, unit keys, package ids, and package keys,

[GHC]

#11015: Add note describing unit ids, unit keys, package ids, and package keys,
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        Reporter:  bgamari           |                Owner:  ezyang
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.11
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 Perhaps `Packages.hs` or `Modules.hs`?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11015#comment:2>
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