the interface `rocketmq.Producer` can't be implemented outside

```
type Producer interface {
Start() error
Shutdown() error
SendSync(ctx context.Context, mq ...*primitive.Message) (*primitive.SendResult, error)
SendAsync(ctx context.Context, mq func(ctx context.Context, result *primitive.SendResult, err error),
msg ...*primitive.Message) error
SendOneWay(ctx context.Context, mq ...*primitive.Message) error
Request(ctx context.Context, ttl time.Duration, msg *primitive.Message) (*primitive.Message, error)
RequestAsync(ctx context.Context, ttl time.Duration, callback internal.RequestCallback, msg *primitive.Message) error
}
```

the struct `RequestCallback` in package `internal`, so the interface `Producer` can't be implemented outside