Add step-wise caching to src-cli

Created by: mrnugget

This is a sub-ticket of https://github.com/sourcegraph/sourcegraph/issues/19476.

The idea is described in https://hungry-yonath-3c673a.netlify.app/, but the short version:

1. Instead of caching only the result of executing all steps in a repository, we cache the results per step
2. We invalidate the cache for a step only if the step or one of the previous steps definitions changed