letsencrypt: Depend on the new certbot package

Created by: SunilMohanAdapa

Instead of the transitional package 'letsencrypt'.

I believe, there is no need to rename the entire module to certbo since letsencrypt is stil a more apt name for the module. Let's Encrypt is a strong brand that people know. The service is still called Let's Encrypt. Only the automation tool has been renamed to certbot to avoid confusion of the service name.

Fixes #576 (closed).