libupkg: Store export parent pointers directly.

Making users handle the package member directly leads to some fairly
ridiculous and somewhat tricky code.  Resolve them all at package load
time and store pointers directly in the upkg_export structure, which
should help simplify the callers.

At this point, we codify the "exports cannot have packages which are
imports" assumption, which appears to be valid.  It's not obvious how
name lookup could possibly succeed if this were not the case.