go.tools/ssa: display named types package-qualified using types.TypeString.

Details:
- use relative (non-qualified) names in more places
- Member interface now has Package(), RelString() methods.
- (*Function).DumpTo: add "# Package: " header.
- Added sanity checks for String functions.

R=gri, gri
CC=golang-dev
https://golang.org/cl/26380043