Allow repository to be used with ROS/ROS 2

[2b-t]

This pull request allows this repository to be used directly with the popular robotics middleware ROS by providing a manifest package.xml, a simple text file containing its dependencies, that is compatible with ROS as well as ROS 2. Previously this was only possible through wrappers such as darknet_ros or darknet_vendor that use the initial Darknet which is no longer actively maintained. This is related to previous issue #5457.

This change makes integrating Darknet into robotics applications much more convenient. It is sufficient to make the package.xml of any package depending on Darknet with <depend>darknet</depend> and linking to it inside the CMakeList.txt with:

find_package(Darknet REQUIRED)
include_directories(
  ${Darknet_INCLUDE_DIR}
)
link_libraries(Darknet::dark)

Additionally INSTALL_RPATH_USE_LINK_PATH might have to be set.