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>

integration/issue7496_shutdown_linux_test.go

@@ -22,7 +22,7 @@ import (
 
 	"github.com/stretchr/testify/require"
 
-	apitask "github.com/containerd/containerd/v2/api/runtime/task/v3"
+	apitask "github.com/containerd/containerd/api/runtime/task/v3"
 	"github.com/containerd/containerd/v2/pkg/namespaces"
 )