Fix SwiftPM module-map builds when .build/checkouts is symlinked on Xcode 26.5

Projects that cache or symlink .build/checkouts/ (for example with Namespace’s nscloud-cache-action) no longer fail to build on Xcode 26.5 with module map file ... not found. Tuist now anchors module-map and header-search flags for its own tuist-derived/ files on $(PROJECT_DIR) instead of $(SRCROOT), so the generated paths stay inside tuist-derived/ and never traverse the symlinked checkouts/ directory. Builds that use binary cache or selective testing remain portable because the settings hasher ignores the substituted absolute path. Local projects and paths that legitimately live inside SwiftPM checkouts are unchanged.