Parsing a URDF string injects "<urdf-string>" into file paths

Environment

• OS Version: Ubuntu 20.04
• Source build sdf12 branch

Description

• Expected behavior: File pathing is not arbitrarily broken via injected string data
• Actual behavior: "<urdf-string>" is prepended to relative paths in URDF file

It would be useful to at least have a config switch suppressing this behavior.

Steps to reproduce

1. Launch ign gazebo
2. Process a URDF file with meshes through xacro
3. Launch robot state publisher to load the output of (2) on a topic
4. Launch ros_ign_gazebo create to spwan an entity, referencing the topic in (3)
5. Examine the resulting file paths and note "<urdf-string>" in mesh paths