go/types: include package import path in NamedType.String().

This avoids ambiguity and makes the diagnostics closer to
those issued by gc, but it is more verbose since it qualifies
intra-package references.

Without extra context---e.g. a 'from *Package' parameter to
Type.String()---we are forced to err on one side or the other.

Also, cosmetic changes to exp/ssa:
- Remove package-qualification workaround in Function.FullName.
- Always set go/types.Package.Path field to the import path,
  since we know the correct path at this point.
- In Function.DumpTo, show variadic '...' and result type info,
  and delete now-redundant "# Type: " line.

R=gri
CC=golang-dev
https://codereview.appspot.com/7325051