Add linter rule for models that are only used as spread sources

[johanste]

Clear and concise description of the problem

In general, models that are only referenced in spreads into other models are not things that we want to emit (note: spreading into op parameters is different). We usually "fix" the problem by erasing unreferenced models in our emitters, but that is work that shouldn't be necessary. Having a model also has the downside of a) us having to give it a (good) name and b) requires documentation. Neither of which are that helpful. It also risks introducing ghost breaking change warnings/confuse our generators if the models were not intended for reuse outside the scope of a given spec (i.e. they are not part of the public surface area in a library)

Checklist

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