Support .nib resources in SwiftPM packages

Swift Packages that ship compiled .nib resources are now bundled correctly and load without crashing. Tuist now recognizes .nib alongside .xib in the default SwiftPM resource file extensions, so packages without an explicit resources: declaration have .nib files picked up by the auto-generated globs. Compiled .nib is also treated as an opaque directory wrapper, matching SwiftPM and Xcode, so it is added as a single reference instead of being expanded into its inner files, which previously produced a malformed bundle that crashed at runtime when the nib was loaded.