go/vcs: isolate into a nested module

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>

Author: dmitshur

go/vcs/go.mod

@@ -0,0 +1,7 @@
+// Deprecated: This module contains one deprecated package.
+// See the package deprecation notice for more information.
+module golang.org/x/tools/go/vcs
+
+go 1.19
+
+require golang.org/x/sys v0.9.0

go/vcs/go.sum

@@ -0,0 +1,2 @@
+golang.org/x/sys v0.9.0 h1:KS/R3tvhPqvJvwcKfnBHJwwthS11LRhmM5D59eEXa0s=
+golang.org/x/sys v0.9.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=

go/vcs/vcs.go

@@ -16,7 +16,7 @@
 // It needed to be manually kept in sync with upstream when changes were
 // made to cmd/go/internal/get, as tracked in go.dev/issue/11490.
 // By now, it has diverged significantly from upstream cmd/go/internal/get
-// behavior and is not being actively updated.
+// behavior and will not receive any further updates.
 package vcs // import "golang.org/x/tools/go/vcs"
 
 import (