[backport kilted] Make the directory-finding substitutions into a PathSubstitution for / operator (backport #914)

[mergify]

Description

Part of robograph-project/planning#38

Makes the two Substitutions that return a directory into a subclass of PathSubstitution

• LaunchLogDir
• ThisLaunchFileDir

So that one can immediately use the path join operator / with them.

Allows changing:

PathSubstitution(ThisLaunchFileDir()) / 'my_launch.py'
# or
PathJoinSubstitution([ThisLaunchFileDir(), 'my_launch.py'])

into

ThisLaunchFileDir() / 'my_launch.py'

Is this user-facing behavior change?

New user-facing capability, no existing usage changed.

Did you use Generative AI?

No

Additional Information

---

This is an automatic backport of pull request #914 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 0d54476 has failed:

On branch mergify/bp/kilted/pr-914
Your branch is up to date with 'origin/kilted'.

You are currently cherry-picking commit 0d54476.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   launch/launch/substitutions/this_launch_file_dir.py
	modified:   launch/test/launch/substitutions/test_launch_log_dir.py
	modified:   launch/test/launch/substitutions/test_this_launch_file_dir.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   launch/launch/substitutions/launch_log_dir.py

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[ahcorde]

Pulls: #919, ros2/launch_ros#498
Gist: https://gist.githubusercontent.com/ahcorde/d28cf19408b963f82bce1fe11fffa342/raw/c113be24e92aa5d5d817a9daf9a9f4e2b554a488/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: kilted
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/17403

• Linux
• Linux-aarch64
• Linux-rhel
• Windows