Redirect tl_expected to system libexpected-dev when available

[otamachan]

This PR updates tl_expected/expected.hpp to redirect to the system tl/expected.hpp when available

• When libexpected-dev is installed (detected via __has_include), tl_expected/expected.hpp redirects to the system tl/expected.hpp with a deprecation warning
• When libexpected-dev is not available (e.g., Windows), the vendored implementation is used as before
• Users who need the vendored version can opt in with -DUSE_VENDORED_TL_EXPECTED=ON
• Added libexpected-dev as build_export_depend to ensure rosdep installs the system package

Related to

• Redirect to system tl-expected package? #9
• tl_expected and libexpected-dev provide overlapping headers on Ubuntu 24.04 generate_parameter_library#311

[christophfroehlich]

At first place I was wondering why I couldn't see your deprecation warning building RSL against your branch: It picked the tl_expected header from rcpputils. I did not know before that this was packaged there. Should we maybe simply remove it here and let ROS PMC handle this in rcpputils?

Changing the includes from
#include <tl_expected/expected.hpp>
to
#include "tl_expected/expected.hpp"
used that of my local workspace and had the desired deprecation warning.

[otamachan]

Thanks for the insight.
Looking into this, it seems rcpputils/tl_expected/expected.hpp was added to internalize the dependency for point_cloud_transport as a core package (ros2/ros2#1516, https://github.com/ros-perception/point_cloud_transport/pull/48/changes). It's not referenced from any of rcpputils' own public headers — it's only used via #include <rcpputils/tl_expected/expected.hpp> in point_cloud_transport's public headers. So the number of external packages depending on it directly should be small, though including point_cloud_transport headers would still trigger the same header guard conflict.

On the other hand, cpp_polyfills' tl_expected has direct downstream dependents, so it can't simply be removed.

Regarding the deprecation warning not appearing — I'll investigate later.

[christophfroehlich]

Sorry, I tried now to just include rcpputils, and it did not work without adapting the includes to <rcpputils/..> as you pointed out: PickNikRobotics/RSL#157

[christophfroehlich]

Shouldn't we remove USE_VENDORED_TL_EXPECTED and just delete the code?
At least this was my proposal in PickNikRobotics/generate_parameter_library#311 (comment) (open for discussion).

Apart from that, the deprecation warning and redirection works as expected. Tested with PickNikRobotics/generate_parameter_library#322 and joint_trajectory_controller