(EMSUSD-1504) Warning Spam when re-loading mayaUsdPlugin

**Describe the bug**
If the mayaUsdPlugin is re-loaded (either by unloading and loading the plugin or by opening additional maya files that will attempt to load the plugin implicitly), there are almost 400 lines of warning spewage issued to the script editor:

`// Warning: file: /dist/sw/maya/2023.update3/scripts/others/registerPluginResource.mel line 81: Plug-in "mayaUsdPlugin" resource identifier "kButtonSave" already registered, it will be replaced`
`// Warning: file: /dist/sw/maya/2023.update3/scripts/others/registerPluginResource.mel line 81: Plug-in "mayaUsdPlugin" resource identifier "kButtonSet" already registered, it will be replaced`
`// Warning: file: /dist/sw/maya/2023.update3/scripts/others/registerPluginResource.mel line 81: Plug-in "mayaUsdPlugin" resource identifier "kButtonCancel" already registered, it will be replaced...`

This is making it difficult for our users to identify warnings and errors that they may need to address

**Steps to reproduce**
Steps to reproduce the behavior:
1. Open maya
2. Load mayaUsdPlugin.so
3. Unload mayaUsdPlugin.so
4. Re-load mayaUsdPlugin.so
5. Observe the script editor output

**Expected behavior**
No warnings are issued upon subsequent mayaUsdPlugin loads

**Specs (if applicable):**
- Linux EL9
- gcc 9.3.1
- Maya 2023.3
- Maya USD commit SHA dev at 15ce0537
- Internal Dev USD build