This is the first step to transition to a better organized python module
structure. Sadly we can't reuse the `pylightning` module as a namespace module
since having importable things in the top level of the namespace is not
allowed in any of the namespace variants [1], hence we just switch over to the
`pyln` namespace. The code the was under `lightning` will now be reachable
under `pyln.client` and we add the `pyln.proto` module for all the things that
are independent of talking to lightningd and can be used for protocol testing.
[1] https://packaging.python.org/guides/packaging-namespace-packages/
Signed-off-by: Christian Decker <decker.christian@gmail.com>
