If you run

	go test -coverpkg=all fmt

one possible interpretation is that you want coverage for all the
packages involved in the fmt test, not all the packages in the world.
Because coverpkg was previously defined as a list of packages
to be loaded, however, it meant all packages in the world.

Now that the go command has a concept of package notation
being used as a matching filter instead of a direct enumeration,
apply that to -coverpkg, so that -coverpkg=all now has the
more useful filter interpretation.

Fixes #10271.
Fixes #21283.

Change-Id: Iddb77b21ba286d3dd65b62507af27e244865072d
Reviewed-on: https://go-review.googlesource.com/76876
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>