go/gcimporter15: update to match latest changes to std lib gcimporter

Mostly a backport of https://go-review.googlesource.com/27816.
Includes an update to TestIssue13898 to match the std lib gcimporter
test.

Change-Id: I790123a49deb2d52f19e51c84bbc7ee74c99156e
Reviewed-on: https://go-review.googlesource.com/27913
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
5 files changed