Make diff minimal when migrating msg packages from ROS1 to ROS2

[mikaelarguedas]

When porting message packages from ROS1 to ROS2 recently I noticed that the CMake diff is so important that it's hard to see the relevant bits (how to use rosidl API and export dependencies).
It would be great to make the diff minimal for at least the message listing part.
Several possibilities:

• modify ROS1 "add_message_files"/"add_service_files" to take accept an empty DIRECTORY and to be passed a relative path for messages (MyMessage -> "msg/MyMessage")
• provide a utility in ROS 2 to concatenate a subdirectory string (eg msg) to each a list of message names

Both these options should allow to use the same list of messages and reduce the diff to how this list is passed to the message generation API.
There may be other alternatives, these 2 are the first ones that came to mind