Allow parsing packages metadata without filelists

[jan-kolarik]

Since Fedora 40, as filelists are not present in the dnf cache by default, we should consider making it optional also when parsing XML files in createrepo_c, specifically in cr_PkgIterator and the underlying structures.

Several related TODOs are already present in the code in xml_parser_main_metadata_together.c.

[ppisar]

If you mean as optional when updating an existing repository, then yes. If you mean by default to stop producing filelists for a new repository, then it's incompatible change and risk the same issues as with changing a default compression format and I don't think it's a good idea.

[dralley]

They mean that when using the cr_PkgIterator API, you should be able to leave the filelists xml or other xml arguments as null.