Restrict any logics inside `__init__.py`

Side effect inside __init__.py are nightmares.

We need to restrict any kind of python code inside __init__.py except:

1. comments
2. docstrings

Imports should be forbidden. Each separate module should have its own public API.

[sobolevn]

Oh, great!

What I have in mind is: 0. We need to create new error for this types of issues, let it be Z400 (as I reserve Z3xx for classes)

1. We need to create ModuleFileVisiter to check these kinds of features
2. Then we need to pass the filename and the ast tree into this visiter
3. And then check only root-level nodes to be in whitelist (just docs for now)
4. Raise errors if other things will be there

Other things that will be required:

1. integration test
2. noqa example, since sometimes we really need this feature, and we need a way to disable the check for a single file

Thanks for your support!

[sobolevn]

Yes, you can disable WPS113 per all __init__.py files or per line.