WIP: Use std::unique_ptr to store cmTargets in a std::unordered_map

• Don' t use an internals class in cmTarget. Integrate cmTargetInternals into cmTarget
• Use std::unordered_map<std::string, std::unique_ptr<cmTarget>> instead of std::unordered_map<std::string, cmTarget> to store targets in a cmMakefile. This should speed up insert operations because only pointers must be moved around instead of whole cmTarget instances.
• Make cmTarget non copyable/movable
• Move cmTarget types into dedicated header cmTargetTypes.h