Add missing special case for GHC.Prim
The special cases for GHC.Prim are a bit fragile. We were relying on (a) the base package to be already registered and (b) the base package not to be hidden. These used to be true with the old library build system, but aren't true now. I'm still not sure why we aren't seeing failures at the moment, but this patch at least fixes the build with 'setup makefile'.