Fix Bundle.module for static frameworks with Metal buildable folders

Static frameworks whose buildable folders contain only Swift and Metal files now correctly synthesize a companion resource bundle and Bundle.module accessor. Previously, these targets skipped bundle synthesis, causing code like device.makeDefaultLibrary(bundle: .module) to fail with an inaccessible module error. Metal files inside buildable folders are now routed to the resource bundle, matching SwiftPM’s behavior for hosting default.metallib.