Remove barrel imports

Created by: orrin-naylor-instacart

Removing barrel exports significantly improves performance in situation where tree shaking is not possible. EG testing via jest. See https://github.com/OpenAPITools/openapi-generator/issues/12081

Barrel exports are files that import and export a group of files. EG models/index.ts

This does not remove the index files. It just removes the internal dependency on them.

To verify, just make sure all the new imports point to a file that:

1. Exists
2. Contains the item being imported

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