Turtlebot3 packages are ignored by colcon build

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I'm following https://emanual.robotis.com/docs/en/p... for instructions on setting up Turtlebot3 packages on my PC (Ubuntu 18.04, with ROS2 Dashing). Here are the instructions:

$ mkdir -p ~/turtlebot3_ws/src
$ cd ~/turtlebot3_ws
$ wget https://raw.githubusercontent.com/ROBOTIS-GIT/turtlebot3/ros2/turtlebot3.repos
$ vcs import src < turtlebot3.repos
$ colcon build --symlink-install

The vcs import seems to work fine (repos are turtlebot3, turtlebot3_msgs, turtlebot3_simulations, DynamixelSDK, and hls_lfcd_lds_driver). But colcon build --symlink-install results in:

$ colcon build --symlink-install
Starting >>> turtlebot3_example
Starting >>> turtlebot3_teleop
Finished <<< turtlebot3_example [0.71s]                                                 
Finished <<< turtlebot3_teleop [0.92s]          

Summary: 2 packages finished [1.57s]

Why aren't the remaining packages building? I have sourced /opt/ros/dashing/setup.bash and ~/turtlebot3_ws/install/setup.bash with no change in the result of colcon build.

Some info which might help:

$ echo $AMENT_PREFIX_PATH
/opt/ros/dashing

$ ros2 launch turtlebot3_fake_node turtlebot3_fake_node.launch.py
Package 'turtlebot3_fake_node' not found: "package 'turtlebot3_fake_node' not found, searching: ['/opt/ros/dashing']"

$ colcon_cd turtlebot3_example works but

$ colcon_cd turtlebot3_fake_node
Could neither find package 'turtlebot3_fake_node' from '~/turtlebot3_ws' nor from the current working directory.

Thanks!