Change the way macOS app names are located (#32706)

Instead of requiring a name_output.sh, expect a file called
.app_filename in the macos/Flutter directory containing just the name of
the application. The expectation is that the Xcode build will create
that file with a script.

This is not intended as a long-term solution, but it's a substantial
improvement over name_output.sh:
- name_output.sh required constructing the full build output path; this
  made sense when it was coupled with build.sh, but now that the
  decision for where build output goes lives in flutter_tool, that logic
  should as well.
- Changing the name of the application required also updating
  name_output.sh, which is error-prone. With .app_filename, it can be
  created using $PRODUCT_NAME, which means that the usual way of setting
  the application name will automatically update this flow as well.

Part of #30706

packages/flutter_tools/lib/src/project.dart

diff --git a/packages/flutter_tools/lib/src/project.dart b/packages/flutter_tools/lib/src/project.dart
index 0032d10..8fec10a 100644
--- a/packages/flutter_tools/lib/src/project.dart
+++ b/packages/flutter_tools/lib/src/project.dart
@@ -575,15 +575,20 @@
 
   Directory get _editableDirectory => project.directory.childDirectory('macos');
 
+  Directory get _cacheDirectory => _editableDirectory.childDirectory('Flutter');
+
   /// Contains definitions for FLUTTER_ROOT, LOCAL_ENGINE, and more flags for
   /// the Xcode build.
-  File get generatedXcodePropertiesFile => _editableDirectory.childDirectory('Flutter').childFile('Generated.xcconfig');
+  File get generatedXcodePropertiesFile => _cacheDirectory.childFile('Generated.xcconfig');
 
   /// The Xcode project file.
   Directory get xcodeProjectFile => _editableDirectory.childDirectory('Runner.xcodeproj');
 
-  // Note: The name script file exists as a temporary shim.
-  File get nameScript => project.directory.childDirectory('macos').childFile('name_output.sh');
+  /// The file where the Xcode build will write the name of the built app.
+  ///
+  /// Ideally this will be replaced in the future with inpection of the Runner
+  /// scheme's target.
+  File get nameFile => _cacheDirectory.childFile('.app_filename');
 }
 
 /// The Windows sub project