check fails "No (or multiple) ghc rts package is registered"

(Imported from [Trac #785](http://hackage.haskell.org/trac/hackage/ticket/785), reported by @dcoutts on 2011-01-06)

For packages that do not depend indirectly on the rts package (ie that have no dependencies specified at all), then the check in the hsc2hs package for the rts package fails.

    hackRtsPackage index =

      case PackageIndex.lookupPackageName index (PackageName "rts") of

        [(_, [rts])]

           -> PackageIndex.insert rts { Installed.ldOptions = [] } index

        _  -> error "No (or multiple) ghc rts package is registered!!"

The reason is that we only store the transative deps of the package in the saved config, so the rts package is not found.

This is of course not a helpful error message. We should remove this check from this place and add something else elsewhere with a more helpful error message. We could do a sanity check during configure or build (but note that we don't do it anymore when configuring ghc itself due to the odd things ghc does during its own build process).

We should probably have a build warning about missing the base package.