An unexported field of a struct is not visible outside of the package
that defines it, so the package path is implicitly part of the
definition of any struct with an unexported field.

Change-Id: I17c6aac822bd0c24188ab8ba1cc406d6b5d82771
Reviewed-on: https://go-review.googlesource.com/32820
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>