Fix SwiftPM builds with symlinked .build/checkouts on Xcode 26.5+

Fixed a build failure when external SwiftPM dependencies were used with a symlinked .build/checkouts directory (common in CI caches such as Namespace’s nscloud-cache-action). Xcode 26.5+ resolves symlinks before traversing .. segments, which caused Tuist’s emitted module map flags of the form $(SRCROOT)/../../tuist-derived/... to land outside the project directory and fail with ‘module map file … not found’. For module maps and headers under tuist-derived/, Tuist now anchors those flags on $(PROJECT_DIR) so the paths stay inside the derived directory and builds succeed.