go/types: embedded fields can be predeclared types

R=adonovan, r
CC=golang-dev
https://codereview.appspot.com/7376055
go/types: more robust imports

- imported objects don't have position information
- gc exported data contains non-exported objects at
  the top-level, guard against them
- better error message when dot-imports conflict
  with local declarations

R=adonovan, r
CC=golang-dev
https://codereview.appspot.com/7379052