Drop support for format 0 and 1 packs. Version 2 will be different in
that it'll use zlib compression instead of lz4, as the latter isn't
really suitable for core.
If I can make it work, I'll make compression engines pluggable, but
the logic for handling these files is spread out enough it's a little
tricky to get that right.
Test changes are only due to pack files being renamed. The contents of
the packs are the same.