odb/source-loose: move loose source into "odb/" subsystem

odb/source-loose: move loose source into "odb/" subsystem

In subsequent patches we'll be turning `struct odb_source_loose` into a
proper `struct odb_source`. As a first step towards this goal, move its
struct out of "object-file.c" and into "odb/source-loose.c".

This detaches the implementation of the loose object source from the
generic object file code, following the same convention already used by
the "files" and "in-memory" sources.

No functional changes are intended.

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