assertion rewrite does not rewrite `__init__.py` in plugin packages (__init__.py)

[asottile]

Here's a ~minimal example:

# pytest_foo/__init__.py
import pytest


@pytest.fixture
def special_asserter():
    def f(x, y):
        assert x == y
    return f

# conftest.py
pytest_plugins = ['pytest_foo']

# t.py
def test(special_asserter):
    special_asserter(1, 2)

$ python -m pytest -p pytest_foo t.py
============================= test session starts ==============================
platform linux -- Python 3.6.7, pytest-4.6.2, py-1.8.0, pluggy-0.12.0
rootdir: /tmp/x
collected 1 item                                                               

t.py F                                                                   [100%]

=================================== FAILURES ===================================
_____________________________________ test _____________________________________

special_asserter = <function special_asserter.<locals>.f at 0x7f54d53c5ea0>

    def test(special_asserter):
>       special_asserter(1, 2)

t.py:2: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

x = 1, y = 2

    def f(x, y):
>       assert x == y
E       AssertionError

pytest_foo/__init__.py:7: AssertionError
=========================== 1 failed in 0.02 seconds ===========================

Expected:

$ python -m pytest t.py
============================= test session starts ==============================
platform linux -- Python 3.6.7, pytest-4.6.2, py-1.8.0, pluggy-0.12.0
rootdir: /tmp/x
collected 1 item                                                               

t.py F                                                                   [100%]

=================================== FAILURES ===================================
_____________________________________ test _____________________________________

special_asserter = <function special_asserter.<locals>.f at 0x7fdf0c1a7d90>

    def test(special_asserter):
>       special_asserter(1, 2)

t.py:2: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

x = 1, y = 2

    def f(x, y):
>       assert x == y
E       assert 1 == 2

pytest_foo/__init__.py:7: AssertionError
=========================== 1 failed in 0.02 seconds ===========================