Merge: Better Vim autocomplete by scanning all imported modules

The new Vim plugin uses `nitls` to locate all Nit modules relevant to the current directory. This is a nice way to support both complex projects and new incomplete modules.

The recommended usage is to update the list of modules when launching Vim and then do it manually afterwards. You may want to update it after adding an importation and saving the file. It could also be updated on each file save or loading but it gets pretty slow. Actually, the whole system may be too slow for older computers (sorry @R4PaSs) or those running inside a slow VM (sorry @BlackMinou and @MehdiAit).

A nice feature to have, would be to call `nitls` on the standard library when there's not local Nit source file or project. It occurs when creating the first Nit source file in an empty directory. @privat, what would be the best way to do that? Could we have `nitls -M standard`?

Pull-Request: #1134
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>