Fix SwiftPM builds when .build/checkouts is a symlink on Xcode 26.5

External SwiftPM dependencies now build correctly when .build/checkouts is replaced by a symlink, such as with CI cache tools like Namespace’s nscloud-cache-action. Xcode 26.5’s explicit-modules dependency scanner resolves symlinks before opening module map paths, and Tuist’s previous $(SRCROOT)/../../tuist-derived/… flag form traversed the symlink and failed with ‘module map file not found’. Tuist now anchors those module map paths to $(PROJECT_DIR), keeping the path inside the derived directory and avoiding the symlinked checkout tree.