ThinCAWrapper: fix for non-Athena projects and cleanup

Frank Winklmeier requested to merge fwinkl/athena:cawrapper into 23.0

Make the also work in non-Athena projects. Use the full $PYTHONPATH to find possible scripts instead of hard-coding project names. Also do general code cleanup and simplification.

This also avoids having to install as python module.

cc @wlampl

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