Context dart-lang/sdk#48272

Pub is no longer creating .packages file, however the tool still has some tests that are creating a mock one (e.g. https://github.com/flutter/flutter/blob/496049263e4efd68ced664ee2c90db9c93f08571/packages/flutter_tools/test/general.shard/plugins_test.dart). These tests should be updated to instead create a .dart_tool/package_config.json file which is what pub is now generating.