SwiftPM prebuilt paths sanitized for install and generation

Paths decoded from workspace-state.json for SwiftPM precompiled artifacts are now sanitized before being turned into AbsolutePath or RelativePath values, with the existing /private/var normalization reused for dependency, artifact, prebuilt, checkout, and include paths. The prebuilt library directory is also added to LD_RUNPATH_SEARCH_PATHS, so macro executables can locate Swift compatibility dylibs such as libswiftCompatibilitySpan.dylib at runtime.