cmd/go/internal/test: add dep from test pkg build to real pkg build

If we have to build a test package, and if the full set of packages
being tested imports the regular package somewhere, then make building
the test package depend on building the regular package.  That way if
the regular package fails to build we only report the error once.

Fixes #44624

Change-Id: Ic7d66d8fec9c4688d369153a4b21194989f8def3
Reviewed-on: https://go-review.googlesource.com/c/go/+/365215


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>