This avoids concurrent runtime package lookups.

Updates #15756

Change-Id: I9e2cbd042aba44923f0d03e6ca5b4eb60fa9e7ea
Reviewed-on: https://go-review.googlesource.com/40853
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>