[MAYA-127035] export with convertMaterialsTo=['rendermanForMaya'] does NOT export renderman material plugs' input connections

**Describe the bug**
Export of renderman materials plugged into the default shadingEngine plugs with shadingMode='useRegistry' and convertMaterialsTo=['rendermanForMaya'] does NOT export the renderman materials' input connections

**Steps to reproduce**
1. Create an object and assign it a renderman material (e.g. PxrMarschnerHair)
2. Give the renderman material some input connections, like a texture for it's diffuseColor plug
3. Connect the renderman material's 'Rman Surface' output into it's shadingEngine's 'Surface Shader' plug
4. export the object with the assigned renderman material
cmds.mayaUSDExport(file=output_filepath, selection=True, shadingMode='useRegistry', convertMaterialsTo=['rendermanForMaya'])
5. inspect the usd file - the renderman shader's Id is correctly identified, but it's inputs are NOT exported

**Expected behavior**
the renderman shader's input connection should also be exported if it is connected to the default shadingEngine plug.

**Attachments**
[test_scene.zip](https://github.com/Autodesk/maya-usd/files/9961360/test_scene.zip)

**Specs (if applicable):**
- OS & version centos 7
- Compiler & version gcc 6.3.1
- Maya version Maya 2019u2
- Maya USD commit SHA: 734b6cf7d / tag 0.20.0
- Pixar USD commit SHA: faed18ce6 / tag 21.11