include contract keys in the Scala codegen output plan when using --root

[S11001001]

Fix to #6660: when filtering with --root, contract keys weren't included in the Scala codegen output plan. We fix it by refining #1586, bringing the key types in line with the way types are generally treated in the definition interface types: as the Ty type variable.

Thanks to @SamirTalwar-DA for discovering.

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