Move modules from namespace 'mlx' to 'mlx/traceability'

[JasperCraeghs]

1. The modules no longer clutter the namespace mlx. They have been moved to the subpackage mlx.traceability from which they are importable along with all classes, except for base classes.
   from mlx.traceability import ItemDirective, report_warning

2. Convert the pkg_resources-style namespace package to a native namespace package