Ensure separation of dependencies in Substrate, Cumulus, Polkadot is maintained

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In the monorepo, I propose we ensure separation of dependencies in Substrate, Cumulus, Polkadot is maintained, so these can be independent of each other long term. It feels easy to bring in dependencies across boundaries that will erode the independence of the three projects.

If I understand correctly, this is the dependency tree that should be enforced:

graph TD;
    Cumulus-->Substrate;
    Cumulus-->Polkadot;
    Polkadot-->Substrate;

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I see there is a Substrate specific script that could be repurposed/expanded for Cumulus, Polkadot, and the whole monorepo:

https://github.com/paritytech/polkadot-sdk/blob/43c0c09bcb9a4db0856598dd07357c0b7292bb84/.gitlab/ensure-deps.sh#L16-L23CI

For reference this blog highlights the easy coupling we inevitably will see hidden by the directory structure, but not the dependency graph.