[MAYA-126190] Parented models into skeleton hierarchy don't export

**Describe the bug**
Any parented models into a skeleton hierarchy are ignored at USD export.

Use cases:
* Eye parented to bone in face.
* Hook attached to the end of an arm.

**Steps to reproduce**
Steps to reproduce the behavior:
1. Load attached arm-ball.ma
2. Select group1 in outliner
3. Export Selection to Maya USD. No adjustment of options. Save USDA.
4. Use usdview or look at usda text to see ball at the end of skeleton is not included.

**Expected behavior**
The ball shape to be included in the export.

**Attachments**
Attached is arm-ball.ma which is a skinned cylinder to a 2-bone skeleton with a ball attached to the last joint.

**Specs (if applicable):**
- Windows 10
- Maya 2022.3
- Maya USD Plugin 0.18 (binary installer downloaded from github)

**Additional context**

More about parenting models into skeletal hierarchy.
* https://knowledge.autodesk.com/support/maya/learn-explore/caas/CloudHelp/cloudhelp/2020/ENU/Maya-GettingStarted/files/GUID-12DAD3E2-E4D6-4CAC-BE5A-88EF49074A2B-htm.html

* Maya FBX export properly exports parented models into a skeleton hierarchy.