Change Library specifier to `def`

[andrewkaufman]

Is this a new feature, an improvement, or a change to existing functionality?

Improvement

Feature Description

It was requested that we use def rather than class specified for external library layers. With class, when opening a library layer in a DCC, it can cause user confusion as to whether the layer was invalid.

Additional context or alternatives

No response

Code of Conduct

• I agree to follow mujoco-usd-converter Code of Conduct
• I have searched the open issues and have found no duplicates