cmd/dist: ignore packages with no Go files in BenchmarkAll
This case recently started happening on the builders.
The synctest experiment was recently enabled for some targets (CL 642422).
This caused the list of standard packages to include testing/synctest.
However, BenchmarkAll tests for all configurations;
some did not include testing/synctest. That caused the test to crash.
Change-Id: Icade10af147c2e2bcbac25bf744919083db3e70f
Reviewed-on: https://go-review.googlesource.com/c/go/+/642397
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Russ Cox <rsc@golang.org>