Proposal: improve the project directory structure

[johnbanq]

To ease cross mod API reference and give a decent project structure to developers new to C++ in general. I think the project directory structure of the sample can be improved:

/
-include
--Mod Name
---HeaderA.h
---HeaderB.h
---HeaderC.h
---etc...
-src
--SourceA.cpp
--SourceB.cpp
--SourceC.cpp
-CMakeFiles.txt

The developers can then reference the headers files like #include<Mod Name/HeaderA.h> etc in this mod or other mods that depends on it.

Expected Changes

we need to update this project to use the directory structure above.
And add target_include_directories the include directory in defmod for the #include<Mod Name/HeaderA.h>
And change the location for scanning .cpp files, perhaps recursive scan should be supported？