Merge branch 'ps/odb-alternates-object-sources'

Code refactoring around alternate object store. * ps/odb-alternates-object-sources: odb: write alternates via sources odb: read alternates via sources odb: drop forward declaration of `read_info_alternates()` odb: remove mutual recursion when parsing alternates odb: stop splitting alternate in `odb_add_to_alternates_file()` odb: move computation of normalized objdir into `alt_odb_usable()` odb: resolve relative alternative paths when parsing odb: refactor parsing of alternates to be self-contained