CMakeLists.txt and package.xml files of ros image to cv image conversion

edit

I have successfully built a node that can convert ros images to cv images based on tutorial given here. I had the following lines in my CMakeLists.txt :

find_package(OpenCV REQUIRED)

include_directories(
  ${catkin_INCLUDE_DIRS}
  ${OpenCV_INCLUDE_DIRS}
)

target_link_libraries(roscv_conv_node
   ${catkin_LIBRARIES}
  ${OpenCV_LIBRARIES}
)

and the following in the package.xml :

<build_depend>OpenCV</build_depend>
<run_depend>OpenCV</run_depend>

The link given above says that opencv2 is to be added in package.xml. Out of curiosity, I removed all the above lines with the word OpenCV in both CMakeLists.txt and package.xml. I ran the catkin_make to compile my node again. It ran successfully and the node works fine too. I think it shouldn't. How is it able to know that the package requires opencv?