Look into using a simpler library namespace

[nrusch]

The Pixar namespace makes for some verbose symbols when it comes to debugging, so it might be nice to choose something more concise.

[nrusch]

This is more complicated than it seems at first glance due to the fact that HdNuke declares its own public tokens, but at least one of the Tf macros for declaring/defining static tokens references unqualified symbols from the USD namespace (namely, TfStaticData and TfToken). Thus, either those symbols would have to be pulled into the HdNuke namespace, or the HdNuke tokens would have to be defined "over the wall" in the USD namespace as a one-off. Neither approach is particularly elegant, so I've punted on this for now.