Merge branch 'ps/odb-source-loose' into ps/odb-source-packed

* 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