enable to run rosstack depends for wetpackages

[k-okada]

with out this PR,

        $ rospack depends image_common
        [rospack] Error: no such package image_common

this is ok, but

        $ rosstack depends image_common
        [rosstack] Error: stack 'image_common' depends on non-existent package 'camera_calibration_parsers' and rosdep claims that it is not a system dependency. Check the ROS_PACKAGE_PATH or try calling 'rosdep update'

is not an expected behavior. Although this is rosstack command, but should be work with catkin metapackage.

[dirk-thomas]

Thanks for the patch.