cmd/go: report which patterns match each package in list

It's important for some uses of go/packages, as well as for some
of go/packages's internal use, to be able to tell which results from
go list output correspond to which patterns, keeping in mind that
a single package might have been matched by multiple patterns.

Also adds test for #26925.

Change-Id: I708ac162f65d9946fe6afb244b08dc7b04d2b530
Reviewed-on: https://go-review.googlesource.com/129060


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>