[GHC] #10117: Change the scheme for reporting redundant imports

[GHC]

#10117: Change the scheme for reporting redundant imports
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        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.8.4
      Resolution:                    |             Keywords:  deprecate
                                     |  warning
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by crockeea):

 I thought I might take a crack at #12067, but I'm afraid those changes
 might conflict with changes from this ticket. Can someone with a better
 knowledge of the codebase weigh in on this?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10117#comment:10>
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