Main module headers are moved 1 level up

e.g. <opencv2/core/core.hpp> become <opencv2/core.hpp>

Also renamed <opencv2/core/opengl_interop.hpp> to <opencv2/core/opengl.hpp>

modules/CMakeLists.txt

@@ -1,3 +1,5 @@
+add_definition(-D__OPENCV_BUILD=1)
+
 if(NOT OPENCV_MODULES_PATH)
   set(OPENCV_MODULES_PATH "${CMAKE_CURRENT_SOURCE_DIR}")
 endif()