Makes programs like ssadump work on packages using vendored code,
for example net/http.
For golang/go#12278.
Depends on CL 17726 in main repository.
Change-Id: Ibabf564e397044a0f449087124dd96161081baaf
Reviewed-on: https://go-review.googlesource.com/17727
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
Russ Cox