odb/source-loose: drop pointer to the "files" source

odb/source-loose: drop pointer to the "files" source

Now that all callbacks of the loose source operate on `struct
odb_source_loose` directly we no longer have to reach into the "files"
source at all.

Drop this field and update `odb_source_loose_new()` to instead accept
all parameters required to initialize itself. This ensures that the
"loose" backend is a fully standalone source.

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