autodoc crashed when a decorator in mocked module takes arguments
because mock system returns the first argument for the decorator as a
decorated object.
This changes the approach for mocking decorators that remembers
arguments for each decoration, and fetch the latest argument on
generating document.
The Python docs for __import__ recommend using importlib.get_module().
https://docs.python.org/3/library/functions.html#__import__
> Note: This is an advanced function that is not needed in everyday
> Python programming, unlike importlib.import_module().
As importlib.get_module() uses the Python module cache and returns the
module, this also allows simplifying many module cache checks of use of
sys.modules.
importlib.get_module() has been available since Python 3.3.
