Make api a Go sub-module

Allow the api to stay at the same v1 go package name and keep using a
1.x version number. This indicates the API is still at 1.x and allows
sharing proto types with containerd 1.6 and 1.7 releases.

Signed-off-by: Derek McGowan <derek@mcg.dev>

contrib/diffservice/service.go

@@ -19,7 +19,7 @@ package diffservice
 import (
 	"context"
 
-	diffapi "github.com/containerd/containerd/v2/api/services/diff/v1"
+	diffapi "github.com/containerd/containerd/api/services/diff/v1"
 	"github.com/containerd/containerd/v2/core/diff"
 	"github.com/containerd/containerd/v2/core/mount"
 	"github.com/containerd/containerd/v2/pkg/oci"