rejig location of package interfaces for haddock
Formerly, setup haddock invoked haddock with a --use-package option for each prerequisite package, causing haddock to invoke ghc-pkg to get the haddock-interfaces and haddock-html fields for each package. The former is accurate, but the latter is not what you want if your documentation is to be placed on the web. Now setup haddock invokes ghc-pkg itself, but if the --html-location option is given, its argument is expanded for each package and used instead of the haddock-html field. The results are then assembed as --read-interface options for haddock. For example, setup haddock '--html-location=http://hackage.haskell.org/packages/archive/$pkg/latest/doc/html' generates HTML documentation with hyperlinks pointing at the pages on HackageDB.