Fix SwiftPM module-map flag resolution for symlinked .build/checkouts on Xcode 26.5

Tuist-generated external SwiftPM projects now use an absolute derived-directory variable for module-map and header-search flags, instead of relative $(SRCROOT)/../../tuist-derived/... paths. This fixes builds that failed on Xcode 26.5 with module map file ... not found when CI tools such as Namespace’s nscloud-cache-action replaced .build/checkouts with a symlink. The change keeps binary-cache and selective-testing hashes portable across machines by excluding the absolute path from the settings hash.