cmd/go: add support for vet-specific export data

This CL makes it possible for vet to write down notes about one package
and then access those notes later, when analyzing other code importing
that package. This is much like what the compiler does with its own export
data for type-checking, so we call it "vet-export" data or vetx data.

The next CL in the stack makes vet actually use this functionality.

Change-Id: Ic70043ab407dfbfdb3f30eaea7c0e3c8197009cf
Reviewed-on: https://go-review.googlesource.com/108558


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