cmd/go: replace Action.Func with Action.Actor

cmd/go: replace Action.Func with Action.Actor

The interface will let us store actor-specific state in the
interface implementation instead of continuing to grow
the Action struct. In the long term we should remove fields
from the struct that are not used by all Actions.

Change-Id: I8ac89eda7a91d742cee547a1f779e9f254dfd2f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/448356
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>