rosmake can't find package, but it's in ROS_PACKAGE_PATH

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I'm trying to build http://wiki.ros.org/april_tags_node . I did these commands to add the package to ros workspace.

$ cd ~/rosbuild_ws
$ rosws set apriltag --svn https://utexas-ros-pkg.googlecode.com/svn/trunk
$ rosws update apriltag
$ source setup.bash

but "$ rosmake apriltag" gives me this error.

[ rosmake ] rosmake starting...                                                 
[ rosmake ] Packages requested are: ['apriltag']                                
[ rosmake ] Logging to directory /home/calanchue/.ros/rosmake/rosmake_output-20160428-114104
[ rosmake ] Expanded args ['apriltag'] to:
[]                                   
[ rosmake ] WARNING: The following args could not be parsed as stacks or packages: ['apriltag']
[ rosmake ] ERROR: No arguments could be parsed into valid package or stack names.

Weird thing is that $ROS_PACKAGE_PATH surely contains apriltag package.

echo $ROS_PACKAGE_PATH
/home/calanchue/rosbuild_ws/apriltag:/home/calanchue/catkin_ws/src:/opt/ros/indigo/share:/opt/ros/indigo/stacks