The previous change marked the vcs package as deprecated.
This change isolates it into a nested module where it
will be tagged as v0.1.0-deprecated, its last version.
The next change deletes the nested module from the tools repo.

For golang/go#57051.

Change-Id: I0b7df60dbe87c1d97f150e5f8ca10e9d281a9364
Reviewed-on: https://go-review.googlesource.com/c/tools/+/505955
gopls-CI: kokoro <noreply+kokoro@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>