New semantic analyzer: Explain that global variable must be declared at module top level

[JukkaL]

The error message is poor for code like this where we don't initialize or declare a global variable at the module top level:

from typing import List


def foo() -> None:
    global bar
    bar = []  # type: List[str]  # Name 'bar' already defined (possibly by an import)
    bar  # Name 'bar' is not defined

It would be better to be explicit and say something like "Global variable must be declared or initialized at module top level".

Follow-up to #6975.

[ljden]

I have just run in to this and agree that the error messaging is poor. However, it does not always make sense to declare the global variable at the module top level.
An option is to have a flag to ignore these global variable errors such as --ignore-method-defined-globals or whatever seems appropriate