Fixed error in relative imports.

[dimasad]

---

Basic Info

Info	Please fill out this column
Ticket(s) this addresses	#471
Primary OS tested on	Ubuntu 20.04
Robotic platform tested on	None, this is a build-time problem only

---

Description of contribution in a few bullet points

• I changed the relative imports to absolute ones that work on the package itself and when including from other packages.
• Removed $catkin_INCLUDE_DIRS from CMakeLists.txt as this is not needed in ROS2/ament (see https://docs.ros.org/en/foxy/Contributing/Migration-Guide.html).
• Removed the "god... getting this to work in ROS2 was a real pain" because the previous solution didn't get it to fully work. I also believe no notice is needed now because the solution is standard.

Description of documentation updates required from your changes

None, as far as I know.

---

Future work that may be required in bullet points

None, as far as I know.

[SteveMacenski]

Please submit a PR for ros2 branch. That way this will be reflected in all future distributions. I usually backport to galactic myself, but I can use this one instead then!