[GHC] #13545: Remove duplication between lookupSubBndrOcc and lookupExportChild

[GHC]

#13545: Remove duplication between lookupSubBndrOcc and lookupExportChild
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           Reporter:  mpickering     |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           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:
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 `lookupSubBndrOcc` is used in expressions such as `x { y = 5 }` to resolve
 field labels such as `y`.

 `lookupExportChild` is used in export lists such as `T(A, x)` to lookup
 `A` and `x`.

 They contain very similar logic but it is duplicated in two places which
 caused #13528.

 It should be easy to refactor them them to deduplicate the logic.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13545>
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