Merge: Improve loader

This PR improves the loader that can now accepts

* directories of projects: to load all modules of all projects (eg `contrib`)
* root directories of groups: to load all modules of the groups (eg `contrib/benitlux`).
  Previously, this worked if there was no `src` subdirectory)

This works for tools that rely on full_parse (basically all tools except the engines).

~~~sh
$ ./test_test_phase ../lib -q
It works
I have 89 projects
I have 285 modules
I have 1038 classes
For 1366 definitions of classes
I have 7101 methods
For 9098 definitions of methods
~~~

Note that if any files has an error most tools will just stop.

Pull-Request: #1229
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>