When attempting to generate XCode projects it would fail due to the target dependency chain not meeting expectations.
This PR, adds the required dependency infomation so that the XCode generator works.
Move the find_package for python into it.
Recommend adding godot-cpp/cmake to CMAKE_MODULE_PATH and using include( GodotCPPModule ) to use functions.
Add target_doc_sources function to simplify the addition of documentation to a binary.