[GHC] #8489: clean up dependency and usages handling in interface files

[GHC]

#8489: clean up dependency and usages handling in interface files
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              Reporter:  errge       |            Owner:  errge
                  Type:  task        |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:  Template    |          Version:  7.7
  Haskell                            |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Moderate (less
  Unknown/Multiple                   |  than a day)
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by simonpj):

 I believe its the package boundary.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8489#comment:8>
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