Update use of `format_items()` to support new arbitrary fields
Refactor package management to use Composer and packagist

* Packages are installed executing `composer require`
* Packages are removed by deleting the directory
* Packages are listed by executing `composer search wp-cli`
* Removes the previous Git-managed `.yml` directory approach.

@todo Commands are autoloaded but can't be run because `RootCommand::load_command` doesn't know how to find them.

See #424