Fix imports (part 2). Disallow wildcard imports (W0401, W0614)

### Motivation:
Using wildcard imports (`from X import *`) is bad practice for three reasons:
* It pollutes the module namespace with unknown names.
* It dumps in the current module the imports of the *imported module*, without any indication (there is actually such a case in our code).
* It makes the code less self-documented; if there are two wildcard imports in the same module, it is unclear from which module the name is imported.

### Changelog:
* Flesh out all wildcard imports.
* Enable `pylint` to flag wildcard imports (`W0401`) and unused names due to wildcard imports (`W0614`).
* Explicitly import modules previously silently provided by wildcard imports.

Depends on #665.
[PEP 328](http://legacy.python.org/dev/peps/pep-0328/)