Make SwiftPM metadata loader portable across hosts by anchoring relative paths to the scratch directory

Tuist’s SwiftPM graph loader now resolves relative paths inside .build/workspace-state.json and .build/swifterpm/package-info/index.json against the project’s scratch directory, while leaving existing absolute paths untouched. This unblocks caching the .build/ directory across different machines: once the companion swifterpm change ships, metadata files can use scratch-relative paths and still load correctly on another host, with no need to wipe or rewrite .build/ on restore.