queries: use metatable for default arch and substring fields
queries: use metatable for default arch and substring fields

Store default values for the `arch` and `substring` fields of
query objects in the metatable, to avoid storing them in the
actual object for dependency listings, that are persisted in
the manifest.

tarantool/luarocks,tarantool/luarocks,tarantool/luarocks,keplerproject/luarocks,keplerproject/luarocks,keplerproject/luarocks,luarocks/luarocks,keplerproject/luarocks,luarocks/luarocks,luarocks/luarocks