Cached Swift macros whose product name differs from the target name resolve correctly

Cache lookups now accept a Swift macro artifact whose product name differs from the target name (e.g. DependenciesMacrosPlugin target with DependenciesMacros.macro artifact), so packages whose macros are referenced via such a target no longer fall back to sources on every clean build. Genuinely absent macro artifacts now produce a clean cache miss instead of force-unwrapping a missing value.