- Define what it means for two identifiers to be unique.

- The current spec is incorrect about exported
identifiers: for instance, it excluded fields
of non-exported types of exported variables
from being exported. It is easier to leave
the detailed specification away and let the
rest of the spec govern access of exported
identifiers.

- The current spec is incorrect about qualified
identifiers: It simply required that an identifier
be exported to be valid in a qualified identifier.
However, qualified identifiers can only access
exported identifiers declared in the package
block of the imported package.

Fixes #1551.

R=r, rsc, iant, ken
CC=golang-dev
https://golang.org/cl/5711043