Delete stale package_config.json in gclient sync hook (#57195)

On the bots there were old and stale `package_config.json` files hanging around that confuse the `dart format` command. This PR adds a step to the `pub_get_offline.py` glcient sync hook to delete all of these files that are not under version control.

tools/pub_get_offline.py

@@ -127,6 +127,17 @@ def find_unlisted_packages():
   return unlisted
 
 
+def delete_config_files():
+  # Find all package_config.json that are not under version control.
+  gitcmd = ['git', 'ls-files', '-o', '**/.dart_tool/package_config.json']
+  files_to_delete = subprocess.check_output(
+      gitcmd, cwd=ENGINE_DIR, stderr=subprocess.STDOUT, text=True
+  ).splitlines()
+  for file in files_to_delete:
+    print('Deleting %s...' % file)
+    os.remove(os.path.join(ENGINE_DIR, file))
+
+
 def main():
   # Intentionally use the Dart SDK prebuilt instead of the Flutter prebuilt
   # (i.e. prebuilts/{platform}/dart-sdk/bin/dart) because the script has to run
@@ -135,6 +146,10 @@ def main():
       SRC_ROOT, 'flutter', 'third_party', 'dart', 'tools', 'sdks', 'dart-sdk', 'bin'
   )
 
+  # Delete all package_config.json files. These may be stale.
+  # Required ones will be regenerated fresh below.
+  delete_config_files()
+
   # Ensure all relevant packages are listed in ALL_PACKAGES.
   unlisted = find_unlisted_packages()
   if len(unlisted) > 0: