Use GPM instead of the custom multipack tool for management of the monorepo

[klavs]

multipack is not documented and is hard to use. GPM would be a better option. It currently lacks local package linking, but I've already submitted a PR https://github.com/dint-dev/gpm/pull/1

[blaugold]

In case this is still relevant melos might be an option. It's what FlutterFire uses to manage their monorepo, so it should be well maintained.

[klavs]

Migration to melos is being considered: #192