Why are Library dependencies not included for interface libraries?

[lukefrasera]

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.

ament_cmake/ament_cmake_target_dependencies/cmake/ament_target_dependencies.cmake

Lines 127 to 157 in 3f305a3

	if(NOT ARG_INTERFACE)
	target_compile_definitions(${target}
	${required_keyword} ${definitions})
	# the interface include dirs must be ordered
	set(interface_include_dirs)
	foreach(interface ${interfaces})
	get_target_property(_include_dirs ${interface} INTERFACE_INCLUDE_DIRECTORIES)
	if(_include_dirs)
	list_append_unique(interface_include_dirs ${_include_dirs})
	endif()
	endforeach()
	ament_include_directories_order(ordered_interface_include_dirs ${interface_include_dirs})
	# the interface include dirs are used privately to ensure proper order
	# and the interfaces cover the public case
	target_include_directories(${target} ${system_keyword}
	PRIVATE ${ordered_interface_include_dirs})
	endif()
	ament_include_directories_order(ordered_include_dirs ${include_dirs})
	target_link_libraries(${target}
	${optional_keyword} ${interfaces})
	target_include_directories(${target} ${system_keyword}
	${required_keyword} ${ordered_include_dirs})
	if(NOT ARG_INTERFACE)
	ament_libraries_deduplicate(unique_libraries ${libraries})
	target_link_libraries(${target}
	${optional_keyword} ${unique_libraries})
	foreach(link_flag IN LISTS link_flags)
	set_property(TARGET ${target} APPEND_STRING PROPERTY LINK_FLAGS " ${link_flag} ")
	endforeach()
	endif()
	endif()