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

An upcoming change to cmd/go will enable this functionality, which
allows vet to write down information about one package for use by
later invocation of vet that analyze code importing that package.

We've intended to do this for a long time, but the build caching was
necessary to have a decent way to manage the vet-specific export data.

This is also an experiment in building scalable whole-program analyses.
In the long term we'd like to allow other analyses to be invoked this way.

Change-Id: I34e4b70445786b2e8707ff6a0c00947bf1491511
Reviewed-on: https://go-review.googlesource.com/117099


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