support remote .cabal files in cabal info

(Imported from [Trac #804](http://hackage.haskell.org/trac/hackage/ticket/804), reported by @dcoutts on 2011-02-14)

Currently, cabal info supports local .cabal files and remote tarballs, but not remote .cabal files.

The [lscabal](http://hackage.haskell.org/package/lscabal) program supports remote .cabal file target like:

$ lscabal http://code.haskell.org/xmonad/xmonad.cabal

 XMonad

 XMonad.Main

Presumably it would also need a more general query language to pull out the fields that the user is interested in.

Note that lscabal only lists exposed modules and only unconditional ones (e.g. not ones that are platform dependent).