op-deployer: deprecate non-forge artifact setup

Description
With forge now we're requiring the entire folder to be present (with the solidity source files). We still have code in op-deployer that supports loading non-forge compatible artifacts, which would ultimately fail to work.

Start by locating op-deployer's artifact setup and the code paths for non-forge-compatible artifacts. Confirm the forge requirement and identify tests covering artifact loading; done means non-forge setup is deprecated and affected tests reflect the supported folder-based layout.

Written by the indexing model from the issue text.