Build catkin packages with colcon

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Hello all,

in my ROS2 workspace I want to build two packages which use the catkin build system ( https://github.com/KIT-MRT/mrt_cmake_... and https://github.com/fzi-forschungszent... ). From the colcon documentation I take that it should be able to use catkin as underlying build system. However when I run colcon build on the workspace it throws the error:

user@user-VirtualBox:~/ros2_ws$ colcon build
Starting >>> mrt_cmake_modules
--- stderr: mrt_cmake_modules                          
CMake Error at CMakeLists.txt:4 (find_package):
  By not providing "Findcatkin.cmake" in CMAKE_MODULE_PATH this project has
  asked CMake to find a package configuration file provided by "catkin", but
  CMake did not find one.

  Could not find a package configuration file provided by "catkin" with any
  of the following names:

    catkinConfig.cmake
    catkin-config.cmake

  Add the installation prefix of "catkin" to CMAKE_PREFIX_PATH or set
  "catkin_DIR" to a directory containing one of the above files.  If "catkin"
  provides a separate development package or SDK, be sure it has been
  installed.


---
Failed   <<< mrt_cmake_modules  [ Exited with code 1 ]

Summary: 0 packages finished [0.63s]
  1 package failed: mrt_cmake_modules
  1 package had stderr output: mrt_cmake_modules
  10 packages not processed

So cmake is apparently not able to "find" catkin. A colcon info correctly identifies the package as type: ros.catkin. If I do a catkin build in a separate catkin workspace it works. I am using Ubuntu 18.04 with both ros-melodic and ros-crystal installed from the ROS repositories. As requested, some further version information:

user@user-VirtualBox:~/ros2_ws$ colcon version-check
colcon-argcomplete 0.3.2: up-to-date
colcon-bash 0.3.2: up-to-date
colcon-cmake 0.2.8: up-to-date
colcon-core 0.3.18: newer version available (0.3.19)
colcon-defaults 0.2.1: up-to-date
colcon-devtools 0.2.1: up-to-date
colcon-library-path 0.2.1: up-to-date
colcon-metadata 0.2.1: up-to-date
colcon-notification 0.2.6: newer version available (0.2.7)
colcon-output 0.2.3: up-to-date
colcon-package-information 0.2.2: up-to-date
colcon-package-selection 0.2.3: newer version available (0.2.4)
colcon-parallel-executor 0.2.3: up-to-date
colcon-pkg-config 0.1.0: up-to-date
colcon-powershell 0.3.4: up-to-date
colcon-python-setup-py 0.2.1: up-to-date
colcon-recursive-crawl 0.2.0: up-to-date
colcon-ros 0.3.8: up-to-date
colcon-test-result 0.3.1: up-to-date
colcon-zsh 0.3.2: up-to-date

I would be thankful if you could point me in the right direction here.