[infra] Use empty package config for package config generator

Previously, stray package_config.json files could cause the package config generator to fail.

Change-Id: If8dc81169a4f95f871572bde5c883a675ab3276b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/401440
Commit-Queue: Sigurd Meldgaard <sigurdm@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>
Auto-Submit: Alexander Thomas <athom@google.com>
Reviewed-by: Sigurd Meldgaard <sigurdm@google.com>

tools/empty_package_config.json

@@ -0,0 +1,12 @@
+{
+    "copyright": [
+        "Copyright (c) 2024, the Dart project authors. Please see the AUTHORS ",
+        "file for details. All rights reserved. Use of this source code is ",
+        "governed by a BSD-style license that can be found in the LICENSE file."
+    ],
+    "comment": [
+        "Empty package configuration, used to bootstrap generate_package_config.dart"
+    ],
+    "configVersion": 2,
+    "packages": []
+}

tools/generate_package_config.py

@@ -35,6 +35,7 @@ def generate_package_config():
     tools_dir = os.path.dirname(os.path.realpath(__file__))
     process = subprocess.run([
         checked_in_sdk_executable(),
+        '--packages=%s' % os.path.join(tools_dir, 'empty_package_config.json'),
         os.path.join(tools_dir, 'generate_package_config.dart')
     ])
     return process.returncode