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>

diff/walking/plugin/plugin.go

@@ -23,11 +23,12 @@ import (
 	"github.com/containerd/containerd/metadata"
 	"github.com/containerd/containerd/platforms"
 	"github.com/containerd/containerd/plugin"
+	"github.com/containerd/containerd/plugin/registry"
 	"github.com/containerd/containerd/plugins"
 )
 
 func init() {
-	plugin.Register(&plugin.Registration{
+	registry.Register(&plugin.Registration{
 		Type: plugins.DiffPlugin,
 		ID:   "walking",
 		Requires: []plugin.Type{