cmd/link: pre-resolve package reference

Pre-resolve package index references, so it doesn't need to do a
map lookup in every cross-package symbol reference resolution. It
increases the memory usage very slightly (O(# imported packages)).

Change-Id: Ia76c97ac51f1c2c2d5ea7ae34853850ec69ef0a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/253604


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