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.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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