scripts: using Path and PurePath for path handling in zephyr modules

Now using only Path and PurePath in zephyr_modules.py to handle module
processing.
This make the code cleaner as well as remove an issue where a module
name would become an empty string when module path contained trailing
path separators.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>