Simpler substitution for package files

[jdlangs]

Feature request

Feature description

Looking at a launch file tutorial, I was struck by the verbose use of both the PathJoinSubstitution and FindPackageShare substitutions. It would be very easy to simplify this for the end-user with a single FindPackageFile substitution, for example. Is there any reason this wasn't done originally and would such a change be welcomed?

[clalancette]

I don't think there is a specific reason this wasn't originally done. We'd be happy to review a pull request that adds this functionality.

[GlowingScrewdriver]

I've got two questions:

1. Would this substitution be something that takes in just a package name and a launch file name? As in:

PythonLaunchDescriptionSource([
   FindPackageFile ('launch_tutorial', 'example_substitutions.launch.py')
])

2. How appropriate would it be to have a similar feature for Yaml launch files? Just enough capability to allow importing a given launch file from a given package? It would definitely allow for a lot more launch files to be written in Yaml instead of Python.