Handle relative imports in stubs

[MikhailArkhipov]

Since typeshed root is not in the path resolver snapshot, os stub is unable to resolve path submodule since it uses

from . import path as path

This causes

import os
os.path.

not to show any completions. Works in 3.x.

Also, for stubs it may be worth resolving stub module first and then real module if the stub is missing. I.e. if A is a stub, import B should resolve to a stub for B and only if stub for B is not found, resolve to real B. This should improve performance of the analysis.

Test: OsPathMembers, Python 2.7