Split buoy_msgs to pure messages and buoy API

[andermi]

Ouch I think we'll need to split this package (ros2/ros2_documentation#2882):

Calling rosidl_generate_interfaces and ament_python_install_package in the same CMake project does not work.
See this Github issue for more info. It is best practice to instead
separate out the message generation into a separate package.

Originally posted by @chapulina in #20 (comment)

[andermi]

So, I was already thinking about splitting to _py and _cpp

What do you think about this structure? (omitting the common files like CMakeLists.txt)

+-- buoy_interfaces (standard ros2 naming for a package with only messages)
|__ +-- msg
|__ +-- srv
+-- buoy_api_py
|__ +-- examples (move python buoy_examples here)
|__ +-- buoy_api (the python interface.py package)
+-- buoy_api_cpp
|__ +-- examples (move cpp buoy_examples here)
|__ +-- include
|   |__ +-- buoy_api (with interface.hpp)
|__ +-- src

[chapulina]

I like the new structure! I assume all packages will still live in this repo, right?

[andermi]

still live in this repo

Yeah, I think so. would we need/want a meta package for buoy_msgs like ros_ign?

[chapulina]

We could add one, those are cheap.