loose: write loose objects map via their source

loose: write loose objects map via their source

When a repository is configured to have a compatibility hash algorithm
we keep track of object ID mappings for loose objects via the loose
object map. This map simply maps an object ID of the actual hash to the
object ID of the compatibility hash. This loose object map is an
inherent property of the loose files backend and thus of one specific
object source.

Refactor the interfaces to reflect this by requiring a `struct
odb_source` as input instead of a repository. This prepares for
subsequent commits where we will refactor writing of loose objects to
work on a `struct odb_source`, as well.

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