pkg: remove ROS boilerplate code, instruct user instead.

Signed-off-by: Peter Soetens <peter@thesourceworks.com>
pkg: rework code such that CMakeLists.txt file is in src/

This is a more sane template, which leaves build products in src and
also workes nice for typegen, since the 'src/' part is dropped of the
filename.

Also fixed the case where the user specifies multiple args on the cmd
line.

Signed-off-by: Peter Soetens <peter@thesourceworks.com>