finds mangled globals and freevars undefined/unused

*Please describe how you installed Flake8*

Example:

```
python3 -m venv ~/.virtualenvs/flake8
workon flake8
pip install -U pip setuptools wheel
pip install flake8 flake8-commas
```

*Please provide the exact, unmodified output of `flake8 --bug-report`*

nah

*Please describe the problem or feature*
flake8 thinks that mangled globals or freevars are undefined in:

```python
def _Ham__spam(value):
     value + ' yes mangling really works like this'

class Ham:
     def foo(self, value):
        return __spam(value)
```

*If this is a bug report, please explain with examples (and example code) what you expected to happen and what actually happened.*

I expected __spam to be considered defined, because it is.