Allow .msg/.srv files to be in subdirectories of msg/, srv/

[janstrohbeck]

If one has the following ROS1 message layout:

msg/
  group1/
    MessageDefinition1.msg
    MessageDefinition2.msg
  group2/
    MessageDefinition3.msg

That is, the .msg files are not directly in the msg folder, but in subfolders. Then the bridge does not find the message files correctly. The same goes for files in subfolders in the srv directory. This PR expands the search for .msg and .srv files to subdirectories, to make it work.

[gbiggs]

@janstrohbeck You wouldn't happen to have a test of a minimal reproducible example I can use to verify this works, would you?