better non-virtual metaIndex.LocalFileName() implementation

better non-virtual metaIndex.LocalFileName() implementation

We can't add a new virtual method without breaking the ABI, but we can
freely add new methods, so for older ABIs we just implement this method
with a dynamic_cast, so that clients can be more ignorant about the API
here and especially don't need to pull a very dirty trick by assuming
internal knowledge (like apt-get did here).