Refactor: USD: Rename usd.hh to usd_private.hh

This header is for 'internal' USD code only (also needed by the hydra
code), and uses references to the USD library itself, so its previous
name was a bit too generic.

First step to move the main public Blender USD API header (`usd.h`) to cpp.

source/blender/io/usd/CMakeLists.txt

@@ -119,7 +119,7 @@ set(SRC
   intern/usd_skel_root_utils.cc
 
   usd.h
-  usd.hh
+  usd_private.hh
 
   intern/usd_armature_utils.hh
   intern/usd_asset_utils.hh

source/blender/io/usd/hydra/usd_scene_delegate.cc

@@ -17,7 +17,7 @@
 #include "WM_types.hh"
 
 #include "usd.h"
-#include "usd.hh"
+#include "usd_private.hh"
 
 namespace blender::io::hydra {
 

source/blender/io/usd/intern/usd_capi_export.cc

@@ -6,7 +6,7 @@
 
 #include "IO_subdiv_disabler.hh"
 #include "usd.h"
-#include "usd.hh"
+#include "usd_private.hh"
 #include "usd_hierarchy_iterator.hh"
 #include "usd_hook.hh"