Sanitize SwiftPM prebuilt paths so precompiled packages load and link

SwiftPM prebuilt integration now correctly handles paths decoded from workspace-state.json. Control characters in those paths are sanitized before being turned into AbsolutePath or RelativePath, and /private/var normalization is reused across dependency, artifact, prebuilt, checkout, and include paths. The prebuilt library directory is also added to LD_RUNPATH_SEARCH_PATHS, so macro executables can locate back-deployed Swift compatibility dylibs such as libswiftCompatibilitySpan.dylib at runtime.