haskell-conduit-extra: this package doesn't contain any modules, so we cannot have documentation

Apparently, Michael Snoyman made all his code depend on an empty package
just in case it turns out that the extra dependency might come in useful
in the future. Go figure.

pkgs/development/libraries/haskell/conduit-extra/default.nix

@@ -4,6 +4,7 @@ cabal.mkDerivation (self: {
   pname = "conduit-extra";
   version = "1.0.0";
   sha256 = "120c3zay8svdw3b9nqgxlrj45a1d4xf0sijkg367m7hp22szvz8a";
+  noHaddock = true;
   meta = {
     homepage = "http://github.com/snoyberg/conduit";
     description = "Temporary placeholder package";