CLI usage

Run conda declarative --help for details:

usage: __main__.py [-h] [-v] [--no-plugins] [-V] COMMAND ...
__main__.py: error: argument COMMAND: invalid choice: 'declarative' (choose from 'activate', 'apply', 'clean', 'commands', 'compare', 'config', 'create', 'deactivate', 'doctor', 'edit', 'env', 'export', 'info', 'init', 'install', 'list', 'notices', 'package', 'remove', 'rename', 'repoquery', 'run', 'search', 'spawn', 'uninstall', 'update', 'upgrade')