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>

plugins/imageverifier/plugin.go

@@ -22,12 +22,13 @@ import (
 	"github.com/containerd/containerd/pkg/imageverifier/bindir"
 	"github.com/containerd/containerd/pkg/tomlext"
 	"github.com/containerd/containerd/plugin"
+	"github.com/containerd/containerd/plugin/registry"
 	"github.com/containerd/containerd/plugins"
 )
 
 // Register default image verifier service plugin
 func init() {
-	plugin.Register(&plugin.Registration{
+	registry.Register(&plugin.Registration{
 		Type:   plugins.ImageVerifierPlugin,
 		ID:     "bindir",
 		Config: defaultConfig(),