external package mod to package.xml

This would enable simpler generation of package.xml files meant only to be used to build packages. See https://github.com/catkin/catkin_tools/pull/95#issuecomment-56755133 for the initial conversation.
Oh yeah, that makes sense. Maybe it would be clearer if you moved the negation:

> it is likely that the package name will not satisfy the catkin naming conventions.