[GHC] #7969: -ddump-minimal-imports should preserve imports required to implement standalone derived instances

[GHC]

#7969: -ddump-minimal-imports should preserve imports required to implement
standalone derived instances
-----------------------------+----------------------------------------------
Reporter:  dsf               |          Owner:                  
    Type:  bug               |         Status:  new             
Priority:  normal            |      Component:  Compiler        
 Version:  7.6.2             |       Keywords:                  
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown      |      Blockedby:                  
Blocking:                    |        Related:                  
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 If a module contains a "deriving instance Show Foo" declaration, the
 import of Foo's constructors needs to be preserved for the module to
 compile.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7969>
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