Inconsistent relative path behaviour

I've noticed that when including another yml file, some paths are treated as relative paths (target.sources and additional includes) for example, but other paths like fileGroups are treated as absolute paths relative the to the root project spec.

I think if a yml file is included using relativePaths: true (the default), all the paths in the file should be treated as relative ones.

I looked through the code a bit and it should be pretty easy to add fileGroups to support being a relative path by updating Project.pathProperties. There may be other path that should be updated as well.

I could make this change if it'd be welcome. The main issue is it would be a breaking change for any project using fileGroups in an included yml file.

Start by reading Project.pathProperties and the handling of included YAML files with relativePaths enabled. Check how fileGroups and the other path properties are resolved, then verify that paths in included files consistently use the included file's location while preserving root-relative behavior where intended. Document or test the breaking-change implications before considering the work done.