用目录结构表示go包依赖关系
(金庆的专栏 2018.6)
摘自：
https://www.ardanlabs.com/blog/2017/02/package-oriented-design.html
If a package wants to import another package at the same level:
* Question the current design choices of these packages.
* If reasonable, move the package inside the source tree for the package that wants to import it.
* Use the source tree to show the dependency relationships.