Improve hierarchical module name handling in MkExternalCore

It's easier for the External Core parser if MkExternalCore prints
module names like:
   base:GHCziBase
rather than like:
   base:GHC.Base
(which it was doing before.)

So now we z-encode the hierarchical module-name part of a module
name, but don't z-encode the ':'.

I also removed some old comments that don't seem relevant anymore.