Generator: Add support for multiple output files

[imagovrn]

Add support for multiple output files especially to python generator, to output proper package with respected namespaces. Adapt CMake for easier integration into C++

[VeaaC]

One concern:
If the output filenames are not available at cmake configuration time, it would make things much harder.
So we should most likely make sure that the output filenames are deterministically inferable from the input file names (without running a parser, I would like to avoid that during config time).

[boxdot]

I prefer to have a single output file. The rationale is that the code is generated and therefore - not really meant for human editing. Splitting code in multiple files is usually done to enable people to work on different independent parts, i.e. it implies better maintainability. The latter is not important for generated code.

In case of python, they made an unfortunate decision to bind modules to the filesystem.

@imagovrn: If we are not going to work on this, what do you think about closing this issue?

[VeaaC]

We do not support including other files at the moment, so closing until this is needed