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>

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