object-file: introduce `struct odb_source_loose`

object-file: introduce `struct odb_source_loose`

Currently, all state that relates to loose objects is held directly by
the `struct odb_source`. Introduce a new `struct odb_source_loose` to
hold the state instead so that it is entirely self-contained.

This structure will eventually morph into the backend for accessing
loose objects. As such, this is part of the refactorings to introduce
pluggable object databases.

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