Blender Git Loki

USD: move library initialisation from `main()` to USD module

Initialize the USD library when used (instead of at startup), so that
this can happen inside the IO/USD module. This makes calls to the USD
library local to Blender's USD code.

Note that failure to find the USD JSON files will now only be reported
when the USD exporter is used, and not on every startup of Blender.

This is the first step in cleaning up the way Blender patches and
initialises the USD library.

Manifest Task: https://developer.blender.org/T80320