[WIP] Using the new install/update mechanism

This is an experimental test to implement new install/update mechanism mentioned in #7816.
This is a work in process PR. Cask maintainers should just skip it until it's mature.

Here's the transition plan TODO list.

 * [x] move brew-cask into `cmd` folder.
 * [x] create dummy brew-cask formula.
 * [ ] update ci test. (Is it necessary/relevant?)
 * [ ] transition test. (Primary test works well, but more test scenarios needed. Help may be required.)
 * [ ] update cask development tool. As under new mechanism, cask development should be in a separate branch. (Help may be required)
 * [ ] update cask document and README.
 * [ ] update [homebrew core document associated with brew-cask](https://github.com/Homebrew/homebrew/blob/master/share/doc/homebrew/External-Commands.md).
 * [ ] update [homebrew core code associated with brew-cask](https://github.com/Homebrew/homebrew/commit/6651fb886fcd7b1003aac7626005fba259988d9c?diff=unified#diff-b82c90101b7f1236459d3ba357ccc6e0R33).

Any comments are welcome.