Python: split API in reader/writer modules

Python: split API in reader/writer modules

babeltrace package's modules are now:

  common:
    class CTFStringEncoding
    class ByteOrder
    class CTFTypeId
    class CTFScope
  reader: everything related to reading
  writer: CTF writer API

Backward compatibility is ensured by importing the
appropriate modules into the package itself.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>