Spurious import warning for type used in nested quoted annotation

[PeterJCLaw]

This is similar to #290 and #313, but seems to still be an issue.

If you import a type and then use it only within a type annotation which includes a nested quoted part, then pyflakes reports the import as unused.

from queue import Queue  # F401: 'queue.Queue' imported but unused
from typing import Optional

def foo(queue: Optional['Queue[int]'] = None) -> None:
    print(queue)

# Revealed type is 'def (queue: Union[queue.Queue[builtins.int], None] =)'
reveal_type(foo)

This can be worked around by quoting the whole of the type annotation:

def foo(queue: 'Optional[Queue[int]]' = None) -> None:
    print(queue)

I'm using pyflakes 2.1.1 on Python 3.5.2 on Linux.

[asottile]

When fixing this, be careful not to break Literal['foo'] (and any other type annotations which require actual strings)

[asottile]

I wrote an implementation of this in #479 -- would appreciate anyone to try it out and review the code \o/