Merge: Honor `source.exclude` in ini files

`source.exclude` is used in very specific cases to prevent some .nit file of directory to belong to a package.

This means two things:
* when the loader is scanning/loading a full package or group, and finds an excluded group or module, then the excluded entity will be not included in the package (and not scanned/loaded)
* when the loader is explicitly requested to load an excluded entity, then it will be detached from the parent package. The excluded modules will be considered as standalone modules.

Pull-Request: #2160
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>