Allow tesseract_collision plugins to be found by ROS2 packages

[schornakj]

This is part of a workaround for an issue in ROS2 pluginlib (ros/pluginlib#161). Briefly, the ROS2 implementation of ClassLoader uses ament_index_cpp::get_package_prefix() when loading plugins, which makes it difficult to load plugins from non-ROS libraries since they aren't added to the AMENT_PREFIX_PATH environment variable and don't have some ROS2-specific config files in their package share directory.

This change solves the second of these problems by writing an empty file to ros2_ws/install/tesseract_collision/share/ament_index/resource_index/packages/tesseract_collision when the package is built.

I think that the first problem will need to be handled separately. So far I've been adding the path to the installed location of tesseract_collision to AMENT_PREFIX_PATH in my launch files.