Add option to select scope for doctests

Original report by asmeurer (@asmeurer?) on Launchpad:

test.py:

import sys

import sys

def func():
    """
    A docstring
    >>> import sys
    >>> sys.path
    """

    return 1

The pyflakes output is

test.py:1: 'sys' imported but unused
test.py:6: redefinition of unused 'sys' from line 1

The line 6 "redefinition of unused 'sys' from line 1" is incorrect, because the doctest scope is separate from the module scope.

I can't figure out how to reproduce it with a small file, but in this file: https://github.com/sympy/sympy/blob/master/sympy/physics/mechanics/functions.py, the output is

sympy/physics/mechanics/functions.py:160: redefinition of unused 'diff' from line 27
sympy/physics/mechanics/functions.py:200: redefinition of unused 'dynamicsymbols' from line 19
sympy/physics/mechanics/functions.py:261: redefinition of unused 'dynamicsymbols' from line 19
sympy/physics/mechanics/functions.py:458: redefinition of unused 'Point' from line 26
sympy/physics/mechanics/functions.py:508: redefinition of unused 'Point' from line 26

That is, it complains about the imports in the doctests, but not the imports themselves (because they actually are used in the file).

Start with the test.py reproducer in the issue and run pyflakes against it, then inspect how doctest code is analyzed. Add a selectable scope for doctests so imports in doctest scope do not produce false redefinition warnings, while genuine unused-import warnings remain; verify the reported output and the SymPy example.