SwiftPM prebuilt paths sanitized, with rpath search added for macros

SwiftPM prebuilt integration stops failing on packages whose workspace-state paths include control characters or /private/var segments: paths sanitized at the same boundary for dependency, artifact, prebuilt, checkout, and include paths. Local macro executables also get prebuilt library directories added to LD_RUNPATH_SEARCH_PATHS so they can locate Swift compatibility dylibs such as libswiftCompatibilitySpan.dylib at runtime.