Add pkgHaddockPath for finding haddock files.
Add pkgHaddockPath for finding haddock files.

bgamari/shaking-up-ghc,sdiehl/ghc,izgzhen/hadrian,sdiehl/ghc,snowleopard/hadrian,sdiehl/ghc,snowleopard/shaking-up-ghc,sdiehl/ghc,sdiehl/ghc,quchen/shaking-up-ghc,sdiehl/ghc,sdiehl/ghc