support module loading (eg. backends) with absolute path

[JBenda]

It would be nice to support this eventually. Because then it's possible to provide a custom module without permission to modify the JULEA installation. Also, this allows compiling modules without a whole JULEA project setup. It's proven practical in projects like MARLIN, also when there has a significant longer build time for their project.

It should be explored if there are security concerns but even without a more privileged JULEA instance it's still practical to only compile the own module without coping them to an undocumented place.