chg `checklinks` to internal command `_checklinks`

* `checklinks` is a very simple implementation useful only for development purposes
* it has no test coverage, only one line of documentation
* users regularly experiment with it and break it, especially
 `brew cask checklinks` with no arguments, which tests all Tapped Casks
Merge pull request #6966 from rolandwalker/checklinks_internal

recast `checklinks` as internal command `_checklinks`