We should add support for older go commands using the multiple-calls
approach of earlier drafts of go/packages.
Also, tag tests for go1.11 to make 1.10 builder happy.
Change-Id: Ia04979528af25cbcd4b4fa5b21cb91d014d530c1
Reviewed-on: https://go-review.googlesource.com/123756
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>