request: pin Melos version in `pubspec.yaml` at workspace root

[blaugold]

Currently, developers install a global version of Melos on their machines, while in CI, the version of Melos is usually pinned.

To better align what version of Melos is used on a per-project basis, Melos could look for a dependency on Melos in a pubspec.yaml at the workspace root and invoke that through dart pub run melos.