change robot structure

mirai-core/src/main/java/net/mamoe/mirai/Robot.java

 package net.mamoe.mirai;
 
 import lombok.Getter;
+import net.mamoe.mirai.contact.Contact;
+import net.mamoe.mirai.message.Message;
 import net.mamoe.mirai.network.RobotNetworkHandler;
 import net.mamoe.mirai.utils.config.MiraiConfigSection;
 

mirai-core/src/main/java/net/mamoe/mirai/message/Message.java

+package net.mamoe.mirai.message;
+
+public class Message {
+}