This gives an easy way to query properties of all the deps
of a set of packages, in a single go list invocation.
Go list has already done the hard work of loading these
packages, so exposing them is more efficient than
requiring a second invocation.

This will be helpful for tools asking cmd/go about build
information.

Change-Id: I90798e386246b24aad92dd13cb9e3788c7d30e91
Reviewed-on: https://go-review.googlesource.com/107776
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>