`default_backend_factories` already decides which store implementations
are the defaults when loading a repo. Defining the initializers there
too keeps that decision in one place for both loading and initializing.
It also means `repo.rs` no longer needs to know about `SimpleOpStore`,
`SimpleOpHeadsStore`, `DefaultIndexStore`, and `DefaultSubmoduleStore`,
which is a step towards making `ReadonlyRepo` independent of the
concrete store implementations.
Assisted-by: Claude Opus 5.5
Martin von Zweigbergk
a2a105b17d