Install package structure. Fix imports (part 1). Enable pylint flag F0401.

### Motivation:
The repo contains at least two python folders that need to import from each other: `db` and `test`. Since relative imports are not currently used, there is a hackish system in place to ensure the folders can see each other (aka messing with `sys.path`) . The current PR fixes this by installing package structure on these folders (actually, a bunch of `__init__.py` files), and replacing the old hacks with proper relative imports. There's also some import cleanup, and a new pylint flag activated to catch import errors in the future: `F0401`.

### Changelog:
* Install subpackage structure in `db` and `test` folders.
* Replace import hacks with proper relative imports.
* Fix broken imports in `features` and `third-party-tools` folders.
* Activate `pylint` import checks: `F0401`.
* Do not `pylint` third party code.
* Suppress pylint import error on `distutils.util`. The package is highly system-dependent and I couldn't get it properly installed on Travis CI.