GHC.Base SOURCE-imports GHC.Err, and re-exports 'error'.  So 
other modules need only import GHC.Base.

This doesn't change the fact that these other modules are all compiled
before GHC.Err, so they are all part of the module loop that starts with
GHC.Base and finishes with GHC.Err.  But it does reduce the occurrence
of those SOURCE imports.