Opt-in SwifterPM install resolution

You can opt in to a SwifterPM-backed path for tuist install and tuist update by setting TUIST_USE_SWIFTERPM. When enabled, Tuist uses SwifterPM for dependency resolution instead of swift package, and tuist generate reuses the cached package-info JSON that SwifterPM writes under .build/swifterpm/package-info when it is fresh and valid. The default behavior continues to use SwiftPM.