Houdini 20.5: expose MoonRay Sdr Python stubs

[gripeyes]

This supports OpenMoonRay/openmoonray#228.

Summary:

• Installs the MoonRay Sdr parser/discovery Python stub packages alongside the native plugins.
• Adds sitecustomize.py so Houdini's regular pxr package can see MoonRay's pxr.* stub modules.
• Keeps the native parser/discovery plugins as the implementation path; the Python files only satisfy Houdini/PXR module discovery.

Validation:

• Sourced the OpenMoonRay Houdini setup.
• Verified Houdini Python can import pxr.MoonrayShaderParser and pxr.MoonrayShaderDiscovery after setup.