[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,
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:  ezyang
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.11
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 It would be great if there were a nice comprehensive Note (perhaps in
 `Module.hs`) describing the various identifiers which we have for modules,
 packages, units, etc.

 These might include,

  * `Package`
  * `PkgName`
  * `PkgKey`
  * `PkgId`
  * `UnitId`
  * `UnitKey`
  * Component?

 After writing this I found [[Commentary/Packages/Concepts]] but it would
 be great if this information found its way into a nice, central Note.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11015>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler