Forward references to type variables behave strangely in fine grained

[ilevkivskyi]

This is similar to #4615

In the situation with a forward reference to type variable, first and second run in fine grained mode give different results:

[case testForwardTypeVar]
import a
[file a.py]
from typing import Generic, TypeVar
import b

def f() -> None:
    class C(Generic[T]): pass
    x: C[int]

T = TypeVar('T')
[file b.py]
y = 0
[file b.py.2]
y = ''
[out]
a.py:5: error: Free type variable expected in Generic[...]
a.py:6: error: "C" expects no type arguments, but 1 given
==

The errors don't appear in the second run. A possible solution is to allow forward references to type variables, although it is quite hard.

A general comment about fine grained mode is that it will be probably hard to support detection of invalid forward references (see for example #2400), the error would depend on the order of targets in file.