Move prebuilt libraries config to distinct file.

Now build system looks more maintanable.
I test with mac, going to test with other platforms.

* All prebuilt libs config moved to
   cmake/Modules/CocosUsePrebuiltLibs.cmake
* Wrap plain find_package to our cocos_find_package. It will not try to
   call find_package if USE_PREBUILT_LIBS and <pkg>_FOUND set to true.
* Other small fixes.