Add files from another target’s buildable folder with the new .exception(target:included:) API

You can now include a file from one target’s buildable folder into a different target without creating a duplicate root-level reference. Use the new additive exception form: .exception(target: "AppTests", included: ["SharedStub.swift"]). Tuist generates the appropriate PBXFileSystemSynchronizedBuildFileExceptionSet for the foreign target, so the file appears once inside the folder and compiles into both targets. Cache invalidation also covers these cross-target inclusions.