Compiled .nib resources now work in SwiftPM packages

SwiftPM packages that ship compiled .nib resources (.xib was already supported) are now bundled correctly: the auto-generated resource globs recognize .nib, and a compiled nib wrapper directory is treated as an opaque resource and copied as one reference rather than being unfolded into its inner files, which previously produced a malformed bundle that crashed at runtime when the nib was loaded.