Fix the haddock --html-location= for relative URLs
The previous fix in isue #1407 turned out to be in the wrong place and so applied to both the haddock-html from the ghc-pkg output, and also to the location passed via the --html-location flag. For the --html-location flag it is important that we can use relative URLs like /package/$pkg-$version/docs because that is what we need for hackage. Note that that is a relative URL, it's relative to the server root. However that was being interpreted as a local file path and being given the file:// prefix. So now we move the file:// URL stuff to the right place so it only applies to the location given in the haddock-html field of the package registration.