Consolidate gogo/protobuf dependencies under our own protobuf package

This would make gogo/protobuf migration easier.

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

metadata/sandbox_test.go

@@ -22,9 +22,9 @@ import (
 
 	"github.com/containerd/containerd/errdefs"
 	"github.com/containerd/containerd/protobuf"
+	"github.com/containerd/containerd/protobuf/types"
 	api "github.com/containerd/containerd/sandbox"
 	"github.com/containerd/typeurl"
-	"github.com/gogo/protobuf/types"
 )
 
 func TestSandboxCreate(t *testing.T) {