SwiftPM workspace state can use scratch-relative paths

The SwiftPM graph loader now resolves relative paths inside .build/workspace-state.json and .build/swifterpm/package-info/index.json against the scratch directory. Paths embedded in these metadata files, such as artifact locations, package references, and prebuilt entries, are interpreted correctly when they are stored relative to the project, paving the way for portable .build/ directories across host machines.