Make findings paths relative

[salsolatragus]

Problem: A detector might (accidentally) output absolute paths to findings. This is not a problem as long as the processing of the findings happens on the same machine as the detector ran on, but it presents an issue for portability.

Solution: Since valid findings should always be located in the project version's build path, we could automatically relativize the paths. Either when loading findings for further processing (not that in this case, the project base path might already be different from the findings' base path!) or in a validation step after running the detector (as part of the CLI?).