See https://github.com/search?q=repo%3Aflutter%2Fflutter%20%2F--packages%2F&type=code.

In a workspace, you can no longer map the project directory $P to $P/.dart_tool/package_config.json, as it's possible the actual package configuration is in a parent directory. We should have Dart tooling organically discover what the right package config is, unless it is a forwarding tool and --packages is passed explicitly by the caller.