Uses get_command_class() to check for the presence of a command.

def check_command(self, name, exclude_packages=None, exclude_command_class=None):
    """
    Uses get_command_class() to check for the presence of a command.
    
    """
    return get_command_class(
        name, 
        exclude_packages=exclude_packages,
        exclude_command_class=exclude_command_class) is not None