object-file: get rid of `the_repository` in loose object iterators

object-file: get rid of `the_repository` in loose object iterators

The iterators for loose objects still rely on `the_repository`. Refactor
them:

  - `for_each_loose_file_in_objdir()` is refactored so that the caller
    is now expected to pass an `odb_source` as parameter instead of the
    path to that source. Furthermore, it is renamed accordingly to
    `for_each_loose_file_in_source()`.

  - `for_each_loose_object()` is refactored to take in an object
    database now and calls the above function in a loop.

This allows us to get rid of the global dependency.

Adjust callers accordingly.

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