Many fixes to DLLisation. These were previously covered up because code was
leaking into the import libraries for DLLs, so the fact that some symbols
were thought of as local rather than in another DLL wasn't a problem.

The main problems addressed by this commit are:

1. Fixes RTS symbols working properly when DLLised. They didn't before.
2. Uses NULL instead of stg_error_entry, because DLL entry points can't be
   used as static initialisers.
3. PrelGHC.hi-boot changed to be in package RTS, and export of PrelNum and
   PrelErr moved to PrelBase, so that references to primops & the like
   are cross-DLL as they should be.
4. Pass imports around as Modules rather than ModuleNames, so that
   ModuleInitLabels can be checked to see if they're in a DLL or not.