Fix imports (part 3). Require explicit relative imports (W0403).

### Motivation:
Explicit relative imports make the code more self-documented. They also safeguard against subtle import bugs when multiple modules exist with the same name (e.g. a proper module and a mock). 

### Changelog:
* Make all relative imports explicit.
* Enable `pylint` to flag implicit relative imports (`W0403`).
* Collapse imports whenever possible.

Depends on #673