Missing Package pr2_mechanism_msgs in pr2_mechanism stack

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I'm running ROS Fuerte and Ubuntu 12.04. I downloaded the stack "pr2_mechanism" using the mercurial repository listed here. The command I executed was:

$ hg clone https://kforge.ros.org/pr2mechanism/hg pr2_mechanism

When I try to compile package "pr2_mechanism_diagnostics" within this stack, I get the following error:

Failed to invoke /opt/ros/fuerte/bin/rospack deps-manifests pr2_mechanism_diagnostics [rospack] Error: package/stack pr2_mechanism_diagnostics depends on non-existent package pr2_mechanism_msgs

The package "pr2_mechanism_msgs" should be in the stack "pr2_mechanism" since it appears in the repository's web interface. However, the package is not included when I clone this repository using the command above. The only packages included are:

1. pr2_controller_interface
2. pr2_controller_manager
3. pr2_hardware_interface
4. pr2_mechanism_diagnostics
5. pr2_mechanism_model
6. realtime_tools

Why is pr2_mechanism_msgs missing?