[melodic]: Switch dependency back to python-rosdep.

Prior to #109 , installing
ros-melodic-desktop on Melodic installed the `rosdep` command-line.
After that change, it no longer does.  For backwards compatibility
reasons, revert that change just for melodic.

Signed-off-by: Chris Lalancette <clalancette@openrobotics.org>

package.xml

@@ -30,8 +30,12 @@
 
  <exec_depend condition="$ROS_PYTHON_VERSION == 2">python-catkin-pkg-modules</exec_depend>
  <exec_depend condition="$ROS_PYTHON_VERSION == 3">python3-catkin-pkg-modules</exec_depend>
- <exec_depend condition="$ROS_PYTHON_VERSION == 2">python-rosdep-modules</exec_depend>
- <exec_depend condition="$ROS_PYTHON_VERSION == 3">python3-rosdep-modules</exec_depend>
+ <!-- This package really only depends on python-rosdep-modules, but in order
+ to keep installing the rosdep command-line tool in Melodic we depend on
+ python-rosdep.
+ -->
+ <exec_depend condition="$ROS_PYTHON_VERSION == 2">python-rosdep</exec_depend>
+ <exec_depend condition="$ROS_PYTHON_VERSION == 3">python3-rosdep</exec_depend>
  <exec_depend>ros_environment</exec_depend>
 
  <test_depend condition="$ROS_PYTHON_VERSION == 2">python-coverage</test_depend>