cmd/go: set DepOnly on package when calling go list -deps

Currently .DepOnly is set when go list -test is invoked to help
distinguish those packages that matched the command line spec from those
which are dependencies (of test packages). This is also useful when
calling go list -deps for the same reason.

Change-Id: Ifc0e68dad0fd01355928793ef803691dee5f4f29
Reviewed-on: https://go-review.googlesource.com/112755


Run-TryBot: Paul Jolly <paul@myitcv.org.uk>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>