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>

pkg/nri/plugin/plugin.go

@@ -19,11 +19,12 @@ package plugin
 import (
 	"github.com/containerd/containerd/pkg/nri"
 	"github.com/containerd/containerd/plugin"
+	"github.com/containerd/containerd/plugins"
 )
 
 func init() {
 	plugin.Register(&plugin.Registration{
-		Type:   plugin.NRIApiPlugin,
+		Type:   plugins.NRIApiPlugin,
 		ID:     "nri",
 		Config: nri.DefaultConfig(),
 		InitFn: initFunc,