Forbid to import symbols from a wrong place

[orsinium]

Rule request

Thesis

In python, you can import from a module any symbol that defined for it.

from typing import collections

Let's forbid indirect imports, except facades.

Implementation

We should allow import symbols:

1. From where they are defined.
2. From __init__.py (facade).

Cases, when the facade isn't __init__.py, are rare (do you have examples?).

Reasoning

1. Too many ways to import one thing
2. Difficult to refactor. You remove import something from a module that imported it for internal usage, and suddenly something else is broken.

Moved from #1092 (comment)

[sobolevn]

This is covered by mypy: see implicit_reexport from https://mypy.readthedocs.io/en/stable/config_file.html

We cannot handle this violation, because we cannot know what's behind import something
Since we do not resolve packages.