Resolve swift Pixar cxx ns typealias.

* This will not work for clients, as they will recieve an error when
  attempting to use the Pixar namespace from any swift code, but
  it should resolve the last remaining errors for a stable PixarUSD
  build, and tie up the final loose ends to allow for full support of
  3rd party plugins to be built with SwiftUSD.

Sources/PixarUSD/Pixar.swift

@@ -31,6 +31,8 @@
 import CxxStdlib
 @_exported import pxr
 
+public typealias Pixar = pxrInternal_v0_23__pxrReserved__
+
 /* --- xxx --- */
 
 public extension Pixar