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/healthcheck/service.go

@@ -18,6 +18,7 @@ package healthcheck
 
 import (
 	"github.com/containerd/containerd/plugin"
+	"github.com/containerd/containerd/plugins"
 
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/health"
@@ -30,7 +31,7 @@ type service struct {
 
 func init() {
 	plugin.Register(&plugin.Registration{
-		Type: plugin.GRPCPlugin,
+		Type: plugins.GRPCPlugin,
 		ID:   "healthcheck",
 		InitFn: func(*plugin.InitContext) (interface{}, error) {
 			return newService()