Re: [PATCH v4 02/17] object-store: rename `object_directory` to `odb_source`

> The `object_directory` structure is used as an access point for a single
> object directory like ".git/objects". While the structure isn't yet
> fully self-contained, the intent is for it to eventually contain all
> information required to access objects in one specific location.
>
> While the name "object directory" is a good fit for now, this will
> change over time as we continue with the agenda to make pluggable object
> databases a thing. Eventually, objects may not be accessed via any kind
> of directory at all anymore, but they could instead be backed by any
> kind of durable storage mechanism. While it seems quite far-fetched for
> now, it is thinkable that eventually this might even be some form of a
> database, for example.
>
> As such, the current name of this structure will become worse over time
> as we evolve into the direction of pluggable ODBs. Immediate next steps
> will start to carve out proper self-contained object directories, which
> requires us to pass in these object directories as parameters. Based on
> our modern naming schema this means that those functions should then be
> named after their subsystem, which means that we would start to bake the
> current name into the codebase more and more.
>
> Let's preempt this by renaming the structure. There have been a couple
> alternatives that were discussed:
>
>   - `odb_backend` was discarded because it led to the association that
>     one object database has a single backend, but the model is that one
>     alternate has one backend. Furthermore, "backend" is more about the
>     actual backing implementation and less about the high-level concept.
>
>   - `odb_alternate` was discarded because it is a bit of a stretch to
>     also call the main object directory an "alternate".
>
> Instead, pick `odb_source` as the new name. It makes it sufficiently
> clear that there can be multiple sources and does not cause confusion
> when mixed with the already-existing "alternate" terminology.
>
> In the future, this change allows us to easily introduce for example a
> `odb_files_source` and other format-specific implementations.