Issues with PackagePath and robot visualization

• The PackagePath in the RobotState's description core segment is now valid. I removed the "package_name/" prefix of the relative path.
• The viz::Robot::file() method works differently. It requires a relative path with "package_name/" prefix. I added it there. We might want to change the behavior of viz::Robot::file() in the future or maybe add an overrive for a PackagePath. However, this is not the scope of this PR.

@gao @kartmann fyi