Dashing merge yaml constructors

[Briancbn]

Add support for yaml constructors for dashing onwards distro

Follow-up on #252 (comment)

I don't think the pytests work in ROS2 due to this error

raise PackageNotFoundError(
1: ament_index_python.packages.PackageNotFoundError: "package 'launch' not found, searching: 
['/home/cypressiea/Documents/ros-workspace/ur2_ws/build/xacro/test/test_ament_index']"

@rhaschke could you help release after this, thanks:)

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Update

Address this 'test cannot run locally' issue in #260