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/version/service.go

@@ -21,6 +21,7 @@ import (
 
 	api "github.com/containerd/containerd/api/services/version/v1"
 	"github.com/containerd/containerd/plugin"
+	"github.com/containerd/containerd/plugin/registry"
 	"github.com/containerd/containerd/plugins"
 	ptypes "github.com/containerd/containerd/protobuf/types"
 	ctrdversion "github.com/containerd/containerd/version"
@@ -30,7 +31,7 @@ import (
 var _ api.VersionServer = &service{}
 
 func init() {
-	plugin.Register(&plugin.Registration{
+	registry.Register(&plugin.Registration{
 		Type:   plugins.GRPCPlugin,
 		ID:     "version",
 		InitFn: initFunc,