Some minor wibbling in printing source locations
I found that an imported instance was getting printed with <no location info>. Fixing this pushed me into a bit more refactoring than I intended, but it's all small aesthetic stuff, nothing fundamental. Caused some error message to change as a result. I removed pprDefnLoc from the GHC API because it doesn't seem to be used. Name.pprNamedefnLoc and pprDefinedAt are probably more useful anyway.