go/types: adjust gcimporter to actual gc export data

Unexported field and method names that appear in the
export data (as part of some exported type) are fully
qualified with a package id (path). In some cases, a
package with that id was never exported for any other
use (i.e. only the path is of interest).

We must not create a "real" package in those cases
because we don't have a package name. Entering an
unnamed package into the map of imported packages
makes that package accessible for other imports.
Such a subsequent import may find the unnamed
package in the map, and reuse it. That reused and
imported package is then entered into the importing
file scope, still w/o a name. References to that
package cannot resolved after that. Was bug.

R=adonovan
CC=golang-dev
https://codereview.appspot.com/7307112