Don't hardwire the build path into the Haddock docs

Don't hardwire the build path into the Haddock docs
Formerly, the ghc-pkg was called to get the HTML dirs for other packages, but
of course doing this at *build* time is totally wrong. Now we use a relative
path, just like before. This is probably not perfect, but much better than
before.

As a sidenote: Cabal calls the relevant flag "html-location", ghc-pkg calls the
field "haddock-html", and Haddock itself uses it as part of "read-interface".
Too much creativity is sometimes a bad thing...