cmd/go: search the target for patterns when -mod=vendor is set

This fixes the root cause of the bootstrap failure reported in
https://groups.google.com/d/msg/golang-dev/xcVJDj5GJ84/U0pVnUOnBwAJ.

Updates #30228

Change-Id: I9ce0898922a5aac1f61ceff30319cf88031676c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/167079


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>