Plugin discovery does not support Dart pub workspaces

[madalindk]

flutter-tvos --version: 1.3.1 (Flutter 3.44.2, Dart 3.12.1) macOS: 15 (Sequoia) Xcode: 16.2

Steps to reproduce

1. Set up a Dart workspace with multiple packages (using resolution: workspace in pubspec.yaml, as supported since Dart 3.6)
2. One of the workspace member packages is a tvOS app with plugin dependencies
3. Run dart pub get at the workspace root
4. Run flutter-tvos build tvos from the app directory

Expected behaviour
Plugins are discovered and registered normally.

Actual behaviour
The build completes with no errors, but produces a binary with no plugins registered. No warning is printed.

Possible root cause
_walkPluginDependencies in lib/tvos_plugins.dart reads package_config.json from project.directory/.dart_tool/:

final File packageConfigFile = project.directory
    .childDirectory('.dart_tool')
    .childFile('package_config.json');

In a Dart workspace, dart pub get only writes package_config.json and package_graph.json at the workspace root's .dart_tool. Member packages instead get workspace_ref.json, which points back to the root. Upstream Flutter's findPackageConfig follows this reference automatically, but flutter-tvos does its own file reads and doesn't know about it.