Reorganize package hierarchy to focus problem domain

[tetsuharuohzeki]

Motivation

1. The current directory (package) design is depends on roles of codes.
2. But this bot contains 2 roles.
   • Labeling.
   • Auto-merger
3. The design should focus to their domain. Not roles of codes.

Plan

1. Introduce domain packages
   • labeling or highfive.
   • auto merger or bors
2. Move the domain specific code to there.