把Mock类移到另外的包中，代理规则就变了

我git了源码，jdk改为1.8
把CookerServiceMock改为public，然后运行了java-demo中的SellerServiceTest，正常测试通过

但是当我把CookerServiceMock移到com.alibaba.demo.mock包下，
在SellerServiceTest中加了@MockWith(CookerServiceMock.class)，
然后神奇的事情发生了，should_sell_sandwich测试失败
```
org.opentest4j.AssertionFailedError:
Expected :Fake-Sandwich-Cooker & Faked-Sandwich
Actual :Real-Sandwich-Cooker & Real-Sandwich
```
我查看了日志，hireSandwichCooker和cookSandwich没有used的记录

后来我测试了一下，当我在代码中加入
```
@MockInvoke(targetClass = CookerService.class)
private String prepareSandwich() {
return "prepareSandwich";
}
```
prepareSandwich有used日志

所以代理的规则是怎么样的，什么时候替换是生效，嵌套调用的生效范围是？

Start with java-demo/SellerServiceTest and the CookerServiceMock example, then inspect how @MockWith and @MockInvoke are applied when the mock class moves to com.alibaba.demo.mock. Reproduce should_sell_sandwich and compare the used logs for hireSandwichCooker, cookSandwich, and prepareSandwich. Done means the proxy and nested-call scope are explained and the reported behavior is either corrected or documented.

Written by the indexing model from the issue text.