[backport humble] Make FindPackage substitutions a Path to get operator / (backport #494)

[mergify]

Description

Part of robograph-project/planning#38
Depends on ros2/launch#914

Make FindPackage{X} substitutions a subclass of PathSubstitution to get the path join operator for free.

Lets you change:

PathSubstitution(FindPackageShare('my_pkg')) / 'launch' / 'my_launch.yaml'
# or
PathJoinSubstitution([FindPackageShare('my_pkg'), 'launch', 'my_launch.yaml'])

into

FindPackageShare('my_pkg') / 'launch' / 'my_launch.yaml'

Is this user-facing behavior change?

New user-facing capability, does not affect existing behavior

Did you use Generative AI?

no

Additional Information

---

This is an automatic backport of pull request #494 done by [Mergify](https://mergify.com).

[ahcorde]

Pulls: ros2/launch#917, #496
Gist: https://gist.githubusercontent.com/ahcorde/6b0c4e2fc0bd4cce3b9882fa4a213918/raw/4007dc59ebe6bf5c50364a32b099989ec3334a5e/ros2.repos
BUILD args: --packages-above-and-dependencies launch_ros test_launch_ros launch
TEST args: --packages-above launch_ros test_launch_ros launch
ROS Distro: humble
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/17405

• Linux
• Linux-aarch64
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• Windows