pub will stop producing .packages: dart-lang/pub#2757

Instead rely on the replacement .dart_tool/package_config.json

Currently rolling the above into the sdk (https://dart-review.googlesource.com/c/sdk/+/231101) fails as such:
https://ci.chromium.org/ui/p/dart/builders/try/flutter-engine-linux-try/678/overview