First step towards removing global var importpkg.

Change-Id: Ifdda7c295e5720a7ff2da9baea17f03f190d48fa
Reviewed-on: https://go-review.googlesource.com/38594
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>