cmd/link: emit better error for duplicated definition

Print the packages where the duplicates come from.

Change-Id: Ib3dc9aa0a3f5ddd97b03744be6d01d4bfcb33996
Reviewed-on: https://go-review.googlesource.com/c/go/+/315949


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>