Change syntax for qualified names in External Core

Change syntax for qualified names in External Core

Two changes that make the ext-core code uglier but the parser easier:

- Prefix qualified names with "^" so that we can more easily
distinguish a qualified name:
   ^a:Foo.Bar.quux
from an unqualified name:
   a

- z-encode package names ("ghc-prim" was the culprit.)