cmd/protoc-gen-go: add "packed" encoding tag to fields

Change-Id: I423039d6c9edb50a07781a6e76336ed364f21866
Reviewed-on: https://go-review.googlesource.com/136359
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/cmd/protoc-gen-go/main.go b/cmd/protoc-gen-go/main.go
index d5bef26..e7119f1 100644
--- a/cmd/protoc-gen-go/main.go
+++ b/cmd/protoc-gen-go/main.go
@@ -631,6 +631,9 @@
 	case protoreflect.Repeated:
 		tag = append(tag, "rep")
 	}
+	if field.Desc.IsPacked() {
+		tag = append(tag, "packed")
+	}
 	// TODO: packed
 	// name
 	name := string(field.Desc.Name())