satisfying package dependencies

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I have package that depends on the willow garage pr2_cockpit package. Is there a way to inform a user that this dependency exists if the package is not already installed (besides looking through the output of rosmake or the package manifest file and checking manually)? I tried adding "ros-diamondback-pr2-cockpit" as an external dependency in the manifest file. However, rosdep install for my package failed with the following error:

rosdep install <package_name> Failed to find rosdep ros-diamondback-pr2-cockpit for package <package_name> on OS:ubuntu version:10.04 ERROR: ABORTING: Rosdeps [u'ros-diamondback-pr2-cockpit'] could not be resolved

Any suggestions of a structured way to install external ros packages would be nice. Is rosinstall the only other option?

Thanks for any help.

marc