[tool] Fix --current-package for app-facing packages (flutter#4399)

The new `--current-package` flag was returning `foo` when run in the app-facing package of a federated plugin called `foo`, but `foo` as a package argument is treated as being the entire group, so it was running all for all of the packages in the plugin. This fixes it to return `foo/foo` in that case, which is how the tool targets app-facing packages specifically.

Author: stuartmorgan-g

script/tool/lib/src/common/package_command.dart

@@ -595,7 +595,15 @@ abstract class PackageCommand extends Command<void> {
     // ... and then check whether it has an enclosing package.
     final RepositoryPackage package = RepositoryPackage(currentDir);
     final RepositoryPackage? enclosingPackage = package.getEnclosingPackage();
-    return (enclosingPackage ?? package).directory.basename;
+    final RepositoryPackage rootPackage = enclosingPackage ?? package;
+    final String name = rootPackage.directory.basename;
+    // For an app-facing package in a federated plugin, return the fully
+    // qualified name, since returning just the name will cause the entire
+    // group to run.
+    if (rootPackage.directory.parent.basename == name) {
+      return '$name/$name';
+    }
+    return name;
   }
 
   // Returns true if the current checkout is on an ancestor of [branch].

script/tool/test/common/package_command_test.dart

@@ -433,6 +433,21 @@ packages/plugin1/plugin1/plugin1.dart
         expect(command.plugins, unorderedEquals(<String>[package.path]));
       });
 
+      test('runs only app-facing package of a federated plugin', () async {
+        const String pluginName = 'foo';
+        final Directory groupDir = packagesDir.childDirectory(pluginName);
+        final RepositoryPackage package =
+            createFakePlugin(pluginName, groupDir);
+        createFakePlugin('${pluginName}_someplatform', groupDir);
+        createFakePackage('${pluginName}_platform_interface', groupDir);
+        fileSystem.currentDirectory = package.directory;
+
+        await runCapturingPrint(
+            runner, <String>['sample', '--current-package']);
+
+        expect(command.plugins, unorderedEquals(<String>[package.path]));
+      });
+
       test('runs on a package when run from a package example directory',
           () async {
         final RepositoryPackage package = createFakePlugin(