TypeVar alias in stub does not work

[rchen152]

Pytype cannot parse a stub that aliases typing.TypeVar to something else:

from typing import TypeVar as _TypeVar
T = _TypeVar('T')
def f(x: T) -> T: ...

An error like this is generated:

  [...]
  File "[...]/pytype/pytd/pytd.py", line 831, in LookupItemRecursive
    raise KeyError(item)
KeyError: TypeDeclUnit(name='typing', [...])

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  [...]
  File "[...]/pytype/load_pytd.py", line 266, in resolve_external_types
    raise BadDependencyError(str(e), name) from e
pytype.load_pytd.BadDependencyError: 'No TypeVar in module typing', referenced from 'foo'