Move plugin type definitions to containerd plugins package

The plugins packages defines the plugins used by containerd.
Move all the types and properties to this package.

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

services/introspection/local.go

@@ -28,6 +28,7 @@ import (
 	"github.com/containerd/containerd/errdefs"
 	"github.com/containerd/containerd/filters"
 	"github.com/containerd/containerd/plugin"
+	"github.com/containerd/containerd/plugins"
 	ptypes "github.com/containerd/containerd/protobuf/types"
 	"github.com/containerd/containerd/services"
 	"github.com/google/uuid"
@@ -39,7 +40,7 @@ import (
 
 func init() {
 	plugin.Register(&plugin.Registration{
-		Type:     plugin.ServicePlugin,
+		Type:     plugins.ServicePlugin,
 		ID:       services.IntrospectionService,
 		Requires: []plugin.Type{},
 		InitFn: func(ic *plugin.InitContext) (interface{}, error) {