Use Twoliter.override as the source of truth for publication overrides

In #372, support was added for allowing pubsys to write to an alternative repository when publishing a kit, yet keeping the kit metadata unchanged.

While discussing the change, we decided that the use of `Infra.toml` and environment variables as an additional source of truth was a bit confusing. We should use `Twoliter.toml` and `Twoliter.override` as the source of truth for kit publication in the future.

Start by reading issue #372 and tracing the publication override handling, then compare how Twoliter.toml, Twoliter.override, Infra.toml, and environment variables are used. Done means publication overrides are sourced from Twoliter.toml and Twoliter.override rather than the additional sources; the issue names no files or tests.

Written by the indexing model from the issue text.