Merge branch 'ps/object-source-loose' into ps/object-read-stream

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