Consolidate gogo/protobuf dependencies under our own protobuf package

This would make gogo/protobuf migration easier.

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

sandbox/helpers.go

@@ -19,8 +19,8 @@ package sandbox
 import (
 	"github.com/containerd/containerd/api/types"
 	"github.com/containerd/containerd/protobuf"
+	gogo_types "github.com/containerd/containerd/protobuf/types"
 	"github.com/containerd/typeurl"
-	gogo_types "github.com/gogo/protobuf/types"
 )
 
 // ToProto will map Sandbox struct to it's protobuf definition