Resolve import paths to get plugin symbol prefixes.

Fixes #19534

Change-Id: Ic25d83e72465ba8f6be0337218a1627b5dc702dc
Reviewed-on: https://go-review.googlesource.com/40994
Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>