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>

runtime/restart/monitor/monitor.go

@@ -26,6 +26,7 @@ import (
 	"github.com/containerd/containerd"
 	"github.com/containerd/containerd/namespaces"
 	"github.com/containerd/containerd/plugin"
+	"github.com/containerd/containerd/plugins"
 	"github.com/containerd/containerd/runtime/restart"
 	"github.com/containerd/log"
 )
@@ -52,10 +53,10 @@ type Config struct {
 
 func init() {
 	plugin.Register(&plugin.Registration{
-		Type: plugin.InternalPlugin,
+		Type: plugins.InternalPlugin,
 		Requires: []plugin.Type{
-			plugin.EventPlugin,
-			plugin.ServicePlugin,
+			plugins.EventPlugin,
+			plugins.ServicePlugin,
 		},
 		ID: "restart",
 		Config: &Config{