Do something about wp-cli-ssh

Today I learned that calling `require()` from a class method gives the code inside the file access to `$this`:

https://github.com/x-team/wp-cli-ssh/blob/master/wp-cli-ssh.php

It's an amazing hack, but it's also fragile. It could easily break when `WP_CLI\Runner` is refactored in the future. Or, we end up like Core, where major refactorings are not possible, because one or more popular extensions would break.