1. If a package doesn't have WP-CLI as a dependency, then it can be loaded before WP-CLI.
1. A typical pattern is to call `WP_CLI::add_command()` at the top or bottom of your package file.
1. The `WP_CLI` class doesn't yet exist if WP-CLI isn't yet loaded.
1. PHP will throw a fatal error if a called class doesn't exist.