Merge branch 'ps/object-source-loose' into ps/object-source-management

A part of code paths that deals with loose objects has been cleaned up. * ps/object-source-loose: object-file: refactor writing objects via a stream object-file: rename `write_object_file()` object-file: refactor freshening of objects object-file: rename `has_loose_object()` object-file: read objects via the loose object source object-file: move loose object map into loose source object-file: hide internals when we need to reprepare loose sources object-file: move loose object cache into loose source object-file: introduce `struct odb_source_loose` object-file: move `fetch_if_missing` odb: adjust naming to free object sources odb: introduce `odb_source_new()` odb: fix subtle logic to check whether an alternate is usable