This patch (inspired by Debian) allows ghc-haddock-library to use our
ghc-attoparsec package instead of using a bundled version.

--- a/haddock-library.cabal	2018-09-01 01:22:18.676855884 -0400
+++ b/haddock-library.cabal	2018-09-01 01:25:10.501150260 -0400
@@ -10,7 +10,6 @@
                       itself, see the ‘haddock’ package.
 license:              BSD3
 license-files:        LICENSE
-                      vendor/attoparsec-0.13.1.0/LICENSE
 maintainer:           Alex Biehl <alexbiehl@gmail.com>, Simon Hengel <sol@typeful.net>, Mateusz Kowalczyk <fuuzetsu@fuuzetsu.co.uk>
 homepage:             http://www.haskell.org/haddock/
 bug-reports:          https://github.com/haskell/haddock/issues
@@ -28,7 +27,6 @@
     , containers   >= 0.4.2.1 && < 0.6
     , transformers >= 0.3.0   && < 0.6
 
-  -- internal sub-lib
   build-depends:        attoparsec
 
   hs-source-dirs:       src
@@ -49,42 +47,6 @@
   if impl(ghc </td></tr></table>
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