ImportError on pxr.Gusd.StageCache due to incomplete CMake install rules

[sanikache]

In src/houdini/lib/H_USD/gusd/_gusd/CMakeLists.txt, the install(TARGETS ...) step only installs the compiled plugin binary.
It does not install the accompanying Python source files (__init__.py, treemodel.py, treeview.py) located one directory up, so the pxr.Gusd package is incomplete after install.

Suggested Fix

Add an additional install(FILES ...) rule in the CMakeLists.txt to copy ../__init__.py, ../treemodel.py, and ../treeview.py into python/lib/python3.11/site-packages/pxr/Gusd, alongside the existing plugin install step.

install(FILES
    ../__init__.py
    ../treemodel.py
    ../treeview.py
    DESTINATION python/lib/python3.11/site-packages/pxr/Gusd)