Changed the plugin manager to use the new CLI infra

The plugin manager now uses the CLI infrastructure. As a consequence its interface has changed (which is a breaking change for configuration management tools). The interface now is consistent across the different plugin command:

 - `plugin install [options] <plugin_name>`
 - `plugin uninstall [options] <plugin_name>`
 - `plugin list [options]`

 Also, renamed `plugin` test package to `plugins` to align with the main package

 Closes #7339.