odb: lift object existence check out of the "loose" backend

odb: lift object existence check out of the "loose" backend

Before writing a new loose object we first check whether the object
already exists in any of the sources attached to the object database.
This results in a couple of issues:

  - We have a layering violation, where the source needs to be aware of
    objects stored in any of the other sources.

  - Every backend would have to reimplement this check, which feels
    somewhat pointless.

  - It is not possible to easily write an object into a source in case
    the same object already exists in another source.

Refactor the code and lift up the object existence check from the
"loose" backend into the generic ODB layer. No callers need adjustment
as none of them write via a specific source, but via the ODB layer.

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