Yacht exercise has non-core dependency

[m-dango]

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.

[kotp]

According to instructions, I believe it should be listed in the exercise's cpan file.

[m-dango]

I think all the per-exercise cpanfiles were removed in #685.

We can probably alter the exercise to either use a constant or refactor it.

[m-dango]

The README will also need updating, the test runner no longer looks for per-exercise cpanfiles as they no longer exist:

perl5/.github/workflows/exercises.yml

Line 32 in 0d7e51f

- name: Exercise modules