D4642: localrepo: iteratively derive local repository type

[Yuya Nishihara]

> +    Extensions should wrap these factory functions to customize repository type
> +    creation. Note that an extension's wrapped function may be called even if
> +    that extension is not loaded for the repo being constructed. Extensions
> +    should check if their ``__name__`` appears in the
> +    ``extensionmodulenames`` set passed to the factory function and no-op if
> +    not.

I assume this will be revisited later. I think it's a source of bugs to
relying on extensions to check if they are enabled. It's also cumbersome
to wrap a function referenced from another table.