FindPackageLaunch / IncludePackageLaunch

[emersonknapp]

Description

Using ros2/launch#849 + FindPackageShare, provide a substitution FindPackageLaunch and action IncludePackageLaunch.

Motivation

Let users avoid two annoying bits of special knowledge:

1. The paths for launchfiles
2. The extension/syntax of launchfiles

Change something like:

include:
  file: $(find-pkg-share my_pkg)/launch/something_launch.py

Into

include_pkg_launch:
  pkg: my_pkg
  name: something

or

include:
  file: $(find-pkg-launch my_pkg something)

Design / Implementation Considerations

I think it's just a new Substiution + Action that uses FindPackageShare and FindLaunchfile in combination.

We could do a first pass implementation without FindLaunchfile which would still shorten things by specifying just package and filename rather than having to do path management

Additional Information

No response