Chore: restructure files

[mysticatea]

What is the purpose of this pull request? (put an "X" next to item)

[X] Other, please explain: moving files.

What changes did you make? (Give an overview)

This PR moves files to the new directory structure. The purpose of this PR is to make clear what functionality use which files. Especially, I wanted to reduce the many util files.

The new structure is:

• lib/init ... The files are to implement --init functionality.
• lib/cli-engine ... The files are to implement CLIEngine public API.
• lib/linter ... The files are to implement Linter public API.
• lib/source-code ... The files are to implement SourceCode public API.
• lib/rule-tester ... The files are to implement RuleTester public API.
• lib/rules ... The files are to implement core rules.
• lib/shared ... The files are shared with multiple functionality.

Is there anything you'd like reviewers to focus on?

Is this move nice?

[mysticatea]

Hi. I rebased this PR.

I'd like to discuss the direction.
This PR restructures files based on public API. The primary purpose is to reduce util files which are hard to track who is using it and to make our codebase more understandable.

• Can we go for the restructure files?
• Are there other ideas about the file structure?

[mysticatea]

What this PR does:

• Moves many files.
• Fixes require() paths for the move.
• Updates architecture.md to explain the new directories.
• Updates .eslintrc.js to prevent files to import the internal files of other directories. (Each directory except lib/shared exposes stuff that it intends other directories to use, with index.js.)

[aladdin-add]

good move! LGTM, but a few non-blocking questions. Thanks!