Add xMsg provisioning

Move packages required by xMsg to its own module, to separate xMsg
dependencies from Clara dependencies. Then include the xMsg classes in
the Clara module.
Add initial C++ dependencies

Install CMake, Protobuf and ZeroMQ.
Move srcdir to the toplevel in the YAML file

The srcdir entry will not be just for Clara sources, but for xMsg too.
Add support for xMsg development sources