Add opt-in SwifterPM install resolution

Set TUIST_USE_SWIFTERPM=1 to have tuist install resolve and update dependencies through SwifterPM instead of swift package. When the SwifterPM package-info cache is present, tuist generate reuses the cached manifest data under .build/swifterpm/package-info instead of reloading every Package.swift. If the cache is missing, stale, or unsupported, Tuist falls back to the standard manifest loader.