Why are Library dependencies not included for interface libraries?

When creating interface libraries that depend on other packages ament explicitly doesn't add necessary build dependencies to the target or export target. If you `target_link_libraries` against an interface library depending on other packages you will get build errors. I am curious why the `if(NOT ARG_INTERFACE)` checks are needed? If you remove them the generated targets as well as cmake exports work properly and carry the associated dependencies.

https://github.com/ament/ament_cmake/blob/3f305a3e1e51f2bd4bd4bf5525a1981c34ff5d08/ament_cmake_target_dependencies/cmake/ament_target_dependencies.cmake#L127-L157