Cached fallback discovery and parallel package acquisition

Xcode's fallback search for local Swift package products now reuses successful manifest dumps across graph loads. Manifest and source edits, added or removed files, tool changes and environment changes invalidate the saved answer. Invalid templates still produce a warning and are retried rather than saved as failures.

Xcode and native Swift package workspaces acquire independent pinned Git repositories concurrently before reading their manifests. Acquisition shares Once's CPU and memory admission budget with build execution, including --cpu-limit and --memory-limit. Shared repository locking, offline reuse, pinned revisions and ordinary-fetch fallback remain unchanged.