[Haddock] [haddock] #217: Remove import of Haddock.Types from the backends

[haddock]

#217: Remove import of Haddock.Types from the backends
-------------------+--------------------------------------------------------
Reporter:  hamish  |       Owner:      
    Type:  defect  |      Status:  new 
Priority:  minor   |   Milestone:  2.13
 Version:  2.9.4   |    Keywords:      
-------------------+--------------------------------------------------------
 Replace it with a new exported module (say Documentation.Haddock.Types) so
 that external backends are not broken when types are added.

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Ticket URL: <http://trac.haskell.org/haddock/ticket/217>
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