SwiftPM metadata paths now resolve relative to the scratch directory

Tuist’s SwiftPM graph loader now resolves relative paths in .build/workspace-state.json and .build/swifterpm/package-info/index.json against the project’s scratch directory. This makes Tuist forward-compatible with the upcoming SwiftPM change that emits relative paths, enabling the .build/ directory to be restored from cache across different machines or checkout locations.