This separates the go list specific behavior from the generalised go/packages
loading behaviour, to enable alternate build system back ends.
Change-Id: I07e7649f8f2afc7470a3ee3d0d743cbbcc6f713e
Reviewed-on: https://go-review.googlesource.com/125715
Run-TryBot: Ian Cottrell <iancottrell@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>