50e4e12c2e
... and use it in `engine_tool` to prove everything is working, i.e. on CI and elsewhere. Partial work towards https://github.com/flutter/flutter/issues/147883. Supersedes and closes https://github.com/flutter/engine/pull/51782 (which was a prototype). See also: - https://flutter.dev/go/pub-workspace, the design doc on the feature. - https://github.com/dart-lang/pub-dev/pull/7762, an example PR. I'll probably end up moving the inline docs in `pubspec.yaml` to a `docs/*.md` once that's a thing.