mypy crashes in attempt to process relative import

[AndrewUshakov]

mypy (just cloned from github) crashed trying to parse line "from .a import *" from a file below.

# -*- coding: utf-8 -*-
from .a import *


def func_b(arg=DEFAULT):
    pass
    #print('ab.b.func_b():', arg)
    #func_a(1)

Stack trace is below:

`H:\My Documents\PyCharm\tst_mypy\ab>mypy b.py
Traceback (most recent call last):
  File "C:\Program Files\Python 3.7\Scripts\mypy-script.py", line 11, in <module>
    load_entry_point('mypy==0.650+dev.e09e72775bfeb622e67cab7d5eae0f12ef4adefa', 'console_scripts', 'mypy')()
  File "c:\program files\python 3.7\lib\site-packages\mypy\__main__.py", line 7, in console_entry
    main(None)
  File "c:\program files\python 3.7\lib\site-packages\mypy\main.py", line 92, in main
    res = build.build(sources, options, None, flush_errors, fscache)
  File "c:\program files\python 3.7\lib\site-packages\mypy\build.py", line 155, in build
    result = _build(sources, options, alt_lib_path, flush_errors, fscache)
  File "c:\program files\python 3.7\lib\site-packages\mypy\build.py", line 204, in _build
    graph = dispatch(sources, manager)
  File "c:\program files\python 3.7\lib\site-packages\mypy\build.py", line 2118, in dispatch
    graph = load_graph(sources, manager)
  File "c:\program files\python 3.7\lib\site-packages\mypy\build.py", line 2320, in load_graph
    ancestor_for=st)
  File "c:\program files\python 3.7\lib\site-packages\mypy\build.py", line 1382, in __init__
    assert id or path or source is not None, "Neither id, path nor source given"
AssertionError: Neither id, path nor source given

[ilevkivskyi]

This is a duplicate of #2974