Extension type (interface com.alibaba.dubbo.registry.RegistryFactory) is not an extension, because it is NOT annotated with @SPI!

- [ ] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.
- [ ] I have checked the [FAQ](https://github.com/apache/dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 2.7.4.1
* Operating System version: mac
* Java version: 8

### Steps to reproduce this issue
```java
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE})
public @interface SPI {

/**
* default extension name
*/
String value() default "";

}
```
` ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.registry.RegistryFactory.class);`抛出异常