* Add `@Spi` annotation as the general annotation for SPI definition.
* Add isDefault in @Spi, add loadDefaultInstance and improve loadFirstInstanceOrDefault method, improve test cases
* Add SpiLoaderException class for thrown when something goes wrong while loading Provider
* Rearrange packages of base SPI mechanism

NOTE: this PR contains breaking changes regarding API.

Signed-off-by: yunfeiyanggzq <yunfeiyang@buaa.edu.cn>

- Refactor the slot interface to support prioritized entry
- Add `entryWithPriority` in SphU

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

Signed-off-by: Eric Zhao <sczyh16@gmail.com>