Rename Query2.hs to Qlang.hs

While starting to use the new filter types, I realised that what is
currently implemented is the equivalent of `lib/qlang.py', not
`lib/query.py', since we only deal with data types for now and not the
actual query runtime functionality (RPC, config, etc.).

Let's rename the file to be more consistent with the Python code base.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>