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/failpoint/cmd/containerd-shim-runc-fp-v1/plugin_linux.go

@@ -23,7 +23,7 @@ import (
 	"path/filepath"
 	"strings"
 
-	taskapi "github.com/containerd/containerd/v2/api/runtime/task/v3"
+	taskapi "github.com/containerd/containerd/api/runtime/task/v3"
 	"github.com/containerd/containerd/v2/cmd/containerd-shim-runc-v2/task"
 	"github.com/containerd/containerd/v2/internal/failpoint"
 	"github.com/containerd/containerd/v2/pkg/oci"