[ffigen] [objc] Incorrect code generation for NSPreviewSupport

[brianquinlan]

The generated code is (ffigen 20.0):

// Undefined class 'NSItemProvider$2'.
// Try changing the name to the name of an existing class, or creating a class with the name 'NSItemProvider$2'.
extension NSPreviewSupport on objc.NSItemProvider$2 {
  /// loadPreviewImageWithOptions:completionHandler:
  void loadPreviewImageWithOptions(
    objc.NSDictionary options, {
    required DartNSItemProviderCompletionHandler completionHandler,
  }) {
    objc.checkOsVersionInternal(
      'NSItemProvider.loadPreviewImageWithOptions:completionHandler:',
      iOS: (false, (8, 0, 0)),
      macOS: (false, (10, 10, 0)),
    );
    _objc_msgSend_o762yo(
      object$.ref.pointer,
      _sel_loadPreviewImageWithOptions_completionHandler_,
      options.ref.pointer,
      completionHandler.ref.pointer,
    );
  }

Changing this fixes the issue:

- extension NSPreviewSupport on objc.NSItemProvider$2 {
+ extension NSPreviewSupport on objc.NSItemProvider {

[liamappelbe]

Looks like there's 2 categories being generated with the NSItemProvider name:

extension NSItemProvider on objc.NSURL {

extension NSItemProvider$1 on objc.NSString {

These are conflicting with package:objective_c's NSItemProvider, causing it to be renamed to NSItemProvider$2.

The fix is that we should never rename ObjC imports. We don't need to rename them, because they have an import prefix.