In earlier versions of Go the "go vet" command would run on regular
source files and test files. That was lost in CL74750.  Bring it back.

This required moving a chunk of code from internal/test to
internal/load. The diff looks big but the code is unchanged.

Fixes #23395

Change-Id: Ie9ec183337e8db81c5fc421d118a22b351b5409e
Reviewed-on: https://go-review.googlesource.com/87636
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>