Cabal passes invalid base-url to Haddock on Windows

[sol]

Haddock can read interface files with e.g.

--read-interface=<base-url>,<path-to-haddock-interface-file>

where base-url has to be a a valid URL. On Windows, Cabal passes something like C:\foo\bar\baz as base-url which is not a valid URL. As a result, the generated index.html won't work with Firefox.

This has been originally report at the HP issue tracker, and Mark reported it on Haddock's issue tracker.

[dcoutts]

This change has broken the feature cabal haddock --html-location=URL when you specify the URL as relative. This is what is causing haddock docs on hackage to appear as file:// URLs. We build the docs using --html-location=/package/$pkg-$version/docs. It should to be a relative URL, both because it's the sensible thing to do but also because we need to not force http or https, but work with either.