odb/source-loose: wire up `read_object_info()` callback

odb/source-loose: wire up `read_object_info()` callback

Move `odb_source_loose_read_object_info()` from "object-file.c" into
"odb/source-loose.c" and wire it up as the `read_object_info()` callback
of the loose source. Callers that previously invoked it directly now go
through the generic `odb_source_read_object_info()` interface instead.

The function `read_object_info_from_path()` cannot be moved along with
it because it is still called by `for_each_object_wrapper_cb()`. It is
therefore kept in place, but adjusted to take a loose source to clarify
that it's always operating on this structure.

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