Use go/packages to find and type check packages using export data.
In addition to type checking with export data, this should also enable
cmd/stringer to work correctly when using go modules.
jba: took over CL, tweaked package.Config use.
Change-Id: Ie253378b52fbd909f7194dfd09c039aab63dd8f0
Reviewed-on: https://go-review.googlesource.com/c/126535
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>