Do not detect workspaces as dart packages

sidekick_core/lib/src/dart_package.dart

@@ -27,6 +27,13 @@ class DartPackage {
         return null;
       }
 
+      final workspaceFile = pubspec['workspace'];
+      if (workspaceFile != null) {
+        // Workspace files are not packages
+        // flutter.dev/go/pub-workspace
+        return null;
+      }
+
       // Check for (optional) flutter dependency
       final deps = pubspec['dependencies'] as YamlMap?;
       final withFlutter = deps?.containsKey('flutter') ?? false;

sidekick_core/test/dart_package.dart → sidekick_core/test/dart_package_test.dart

@@ -87,5 +87,24 @@ flutter:
       expect(package!.name, 'package_name');
       expect(package.isFlutterPackage, isTrue);
     });
+
+    test('a workspace is not a DartPackage', () {
+      final tempDir = Directory.systemTemp.createTempSync();
+      addTearDown(() => tempDir.deleteSync(recursive: true));
+      tempDir.file('pubspec.yaml')
+        ..createSync()
+        ..writeAsStringSync('''
+name: _         # Can be anything, _ by convention.
+environment:
+  sdk: ^3.5.0   # Must be ^3.5.0 or later for workspace to be allowed
+workspace:
+- pkgs/app1     # The workspace-packages must be subdirectories
+- pkgs/app2
+- pkgs/shared
+''');
+
+      final package = DartPackage.fromDirectory(tempDir);
+      expect(package, isNull);
+    });
   });
 }