Load an fabsetup addon given by 'package_name' and hook it in the
    base task namespace 'username'.

    Args:
        username(str)
        package_name(str)
        _globals(dict): the globals() namespace of the fabric script.

    Return: None

def load_addon(username, package_name, _globals):
    '''Load an fabsetup addon given by 'package_name' and hook it in the
    base task namespace 'username'.

    Args:
        username(str)
        package_name(str)
        _globals(dict): the globals() namespace of the fabric script.

    Return: None
    '''
    addon_module = get_or_create_module_r(username)
    package_module = __import__(package_name)
    add_tasks_r(addon_module, package_module, package_name)
    _globals.update({username: addon_module})
    del package_module
    del addon_module