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.