fix relative import of mock module

[vivster7]

Fixes #549

Description

from mock import mock is syntactically valid.
Without this patch, this transforms to from unittest.mock import mock (syntactically invalid)
With this patch, this line becomes from unittest import mock

Details

This patch looks for the identifier mock in the names part of the ImportFrom AST node. Then, it handles this case in one of two ways:

1. len(names) == 1.
   a. rewrite the module: mock -> unittest
2. len(names) > 1.
   a. rewrite the module: mock -> unittest.mock
   b. remove the identifier mock from the list of names.
   c. add import on a new line: from unittest import mock

[vivster7]

Looks like I'm failing a coverage test (which is super neat!) -- will fixup and reopen.

[sadikkuzu]

@vivster7
For avoiding those auto-fixes, you can run pre-commit install in your local folder.

[asottile]

via #673