Fix imports (part 1). Enable pylint flag F0401. Install boto as test requirement.

### 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 subpackage 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 imports.
* Activate `pylint` import checks: `F0401`.
* Fix broken imports in `features` and `third-party-tools` folders.
* Install `boto` as test requirement. Purge spurious `requirement.txt` files.