Generalize the plugin package

Remove containerd specific parts of the plugin package to prepare its
move out of the main repository. Separate the plugin registration
singleton into a separate package.

Separating out the plugin package and registration makes it easier to
implement external plugins without creating a dependency loop.

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

services/opt/service.go

@@ -22,6 +22,7 @@ import (
 	"path/filepath"
 
 	"github.com/containerd/containerd/plugin"
+	"github.com/containerd/containerd/plugin/registry"
 	"github.com/containerd/containerd/plugins"
 )
 
@@ -32,7 +33,7 @@ type Config struct {
 }
 
 func init() {
-	plugin.Register(&plugin.Registration{
+	registry.Register(&plugin.Registration{
 		Type: plugins.InternalPlugin,
 		ID:   "opt",
 		Config: &Config{