This will be more robust in the faces of
future changes to the pkg dir layout.

Change-Id: Iaf078093f02ef3a10884a19c25e2068cbbf5f36a
Reviewed-on: https://go-review.googlesource.com/27929Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>