deprecate codegen index.sol pattern

[holic]

I thought this pattern would be nice to simplify importing codegen things from one place, e.g.

import { Tables, ResourceIds, StoreHooks } from "./codegen/index.sol";

but it turns out ANY changes to things imported in this index.sol file, even if its not imported into the user project, will change the resulting bytecode of the contract doing the import.

We should probably deprecate/discourage this pattern for better deterministic deploys. We should also migrate our usages before we tackle #2835.