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/namespaces/local.go

@@ -27,6 +27,7 @@ import (
 	"github.com/containerd/containerd/metadata"
 	"github.com/containerd/containerd/namespaces"
 	"github.com/containerd/containerd/plugin"
+	"github.com/containerd/containerd/plugins"
 	ptypes "github.com/containerd/containerd/protobuf/types"
 	"github.com/containerd/containerd/services"
 	bolt "go.etcd.io/bbolt"
@@ -37,18 +38,18 @@ import (
 
 func init() {
 	plugin.Register(&plugin.Registration{
-		Type: plugin.ServicePlugin,
+		Type: plugins.ServicePlugin,
 		ID:   services.NamespacesService,
 		Requires: []plugin.Type{
-			plugin.EventPlugin,
-			plugin.MetadataPlugin,
+			plugins.EventPlugin,
+			plugins.MetadataPlugin,
 		},
 		InitFn: func(ic *plugin.InitContext) (interface{}, error) {
-			m, err := ic.Get(plugin.MetadataPlugin)
+			m, err := ic.Get(plugins.MetadataPlugin)
 			if err != nil {
 				return nil, err
 			}
-			ep, err := ic.Get(plugin.EventPlugin)
+			ep, err := ic.Get(plugins.EventPlugin)
 			if err != nil {
 				return nil, err
 			}