This CL adds support for accepting package config from
the go command. Paired with CL 74356 this lets us make
sure vet has complete information about package sources.
This fixes many issues (see CL 74356 for the list), including
mishandling of cgo and vendoring.

Change-Id: Ia4a1dce6f9b1b0a8ef5fdf9005a20a8b294969f1
Reviewed-on: https://go-review.googlesource.com/74355
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>