Fixes handling of localized project files

Made changes so synx understands Xcode's weird subfolder structure for localized files and folders (PBXVariantGroup), added a file with multiple localizations to the dummy project, and updated the tests.
Not with localized stuff -- a localized file.txt should actually have paths e.g. en.lproj/file.txt, de.lproj/file.txt, etc, since its parent group has display name "file.txt" but no actual (relative) path, it's just a dummy container. Using the explicit relative path this way guarantees that we always use the full relative path even if it involves extra directories.