[Lean] Rename all references to `Core` to `Core_models`

[abentkamp]

We should rename all references to Core to Core_modules when extracting Lean code to be able to use the autogenerated core modules (coming from PR #1849).

Comment from @maximebuyse :

Having a phase for that would be nice but we could have problems to match against names from Core. In the F* backend we do that in name rendering which works quite well

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[abentkamp]

Done.