[tool] Support third_party for --current-package (#7967)

Fixes `--current-package` so that when run on a package in third_party/packages/ in works as expected, rather than failing with an error message saying that it must be run from inside a package.

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

@@ -662,22 +662,30 @@ abstract class PackageCommand extends Command<void> {
   }
 
   String? _getCurrentDirectoryPackageName() {
-    // Ensure that the current directory is within the packages directory.
-    final Directory absolutePackagesDir = packagesDir.absolute;
+    final Set<Directory> absolutePackagesDirs = <Directory>{
+      packagesDir.absolute,
+      thirdPartyPackagesDir.absolute,
+    };
+    bool isATopLevelPackagesDir(Directory directory) =>
+        absolutePackagesDirs.any((Directory d) => d.path == directory.path);
+
     Directory currentDir = packagesDir.fileSystem.currentDirectory.absolute;
-    if (!currentDir.path.startsWith(absolutePackagesDir.path) ||
-        currentDir.path == packagesDir.path) {
+    // Ensure that the current directory is within one of the top-level packages
+    // directories.
+    if (isATopLevelPackagesDir(currentDir) ||
+        !absolutePackagesDirs
+            .any((Directory d) => currentDir.path.startsWith(d.path))) {
       return null;
     }
-    // If the current directory is a direct subdirectory of the packages
+    // If the current directory is a direct subdirectory of a packages
     // directory, then that's the target.
-    if (currentDir.parent.path == absolutePackagesDir.path) {
+    if (isATopLevelPackagesDir(currentDir.parent)) {
       return currentDir.basename;
     }
     // Otherwise, walk up until a package is found...
     while (!isPackage(currentDir)) {
       currentDir = currentDir.parent;
-      if (currentDir.path == absolutePackagesDir.path) {
+      if (isATopLevelPackagesDir(currentDir)) {
         return null;
       }
     }

script/tool/test/common/package_command_test.dart

@@ -455,6 +455,19 @@ packages/plugin1/plugin1/plugin1.dart
         expect(command.plugins, unorderedEquals(<String>[package.path]));
       });
 
+      test('runs on a package when run from the third_party/packages directory',
+          () async {
+        final RepositoryPackage package =
+            createFakePlugin('a_package', thirdPartyPackagesDir);
+        createFakePlugin('another_package', thirdPartyPackagesDir);
+        fileSystem.currentDirectory = package.directory;
+
+        await runCapturingPrint(
+            runner, <String>['sample', '--current-package']);
+
+        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);