The loose object source has been refactored into a proper `struct
odb_source`.
* ps/odb-source-loose:
odb/source-loose: drop pointer to the "files" source
odb/source-loose: stub out remaining callbacks
odb/source-loose: wire up `write_object_stream()` callback
object-file: refactor writing objects to use loose source
odb/source-loose: wire up `write_object()` callback
loose: refactor object map to operate on `struct odb_source_loose`
odb/source-loose: wire up `freshen_object()` callback
odb/source-loose: drop `odb_source_loose_has_object()`
odb/source-loose: wire up `count_objects()` callback
odb/source-loose: wire up `find_abbrev_len()` callback
odb/source-loose: wire up `for_each_object()` callback
odb/source-loose: wire up `read_object_stream()` callback
odb/source-loose: wire up `read_object_info()` callback
odb/source-loose: wire up `close()` callback
odb/source-loose: wire up `reprepare()` callback
odb/source-loose: start converting to a proper `struct odb_source`
odb/source-loose: store pointer to "files" instead of generic source
odb/source-loose: move loose source into "odb/" subsystem
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