Yacht exercise has non-core dependency

The `yacht` exercise uses the `Readonly` module, which is not perl core. AFAIK this is the only exercise using a non-core module. While the module is available on the test runner, it would have to be installed manually by CLI users.

Start with the `yacht` exercise and inspect how the `Readonly` module is used, then check how dependencies are handled for CLI users and the test runner. Done means the exercise no longer requires a non-core Perl module and still works in both environments.

Written by the indexing model from the issue text.