finds mangled globals and freevars undefined/unused

Original report by tagrain on Launchpad:

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:

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.

Run the provided Python example through pyflakes and confirm how the mangled global or free variable is reported. Trace the name-resolution path that produces the undefined warning, then add a regression test for the example. Done means the valid mangled name is no longer reported as undefined while genuinely undefined names still are.