gocore: fix removal of package paths

They aren't always prefixes.  For instance, map[string]foo/bar.
Do a better job by removing word/ everywhere it appears in a type.

Change-Id: I22d8e00f9eef225dd143cad1db3aa0eeb901ad10
Reviewed-on: https://go-review.googlesource.com/92416
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Peter Weinberger <pjw@google.com>