More resilient SwiftPM dependency resolution via subprocess

Dependency resolution is now performed through a SwiftPM subprocess instead of being linked directly into the Tuist binary. This insulates Tuist from breaking changes in SwiftPM internals, reduces the risk of resolution-related crashes, and removes the raw swift-package-manager source dependency from the shipped binary. The change should also make the resulting Package.resolved file more stable across runs, producing fewer spurious diffs in version control.