Referenced shader attributes issue

[tryptik]

When exporting file nodes from Maya to Usd and the file node is referenced, the export code does not recognize that the Maya attributes for scale/bias are not set properly despite these values being correct in the referenced Maya scene. This seems to be because the exporting code is using MPlug::getSetAttrCmds and checking the result to see if it is empty to determine if the values need to be written.

This P/R checks the Dependency node to see if it is from a referenced file, then uses MPlug::isDefaultValue to determine if the plug values need to be authored on the exported Usd data.

Note:
In my past testing, Plug::isDefaultValue seemed unreliable, and I suspect it remains so based on the code using MPlug::getSetAttrCmds, so if there is a better approach, please advise.

MPlug::isFromReferencedFile does not seem to work, which is why MFnDependencyNode::isFromReferencedFile is used.

Note
This was migrated from a previous P/R that had to be removed - Check if node is referenced - use default value to detect change #2917.

[tryptik]

This change was approved in P/R #2917 and it was recommended that I create a new P/R and will delete the other one.

[pierrebai-adsk]

I noticed too late that this branch is far behind our dev branch. If the build passes, I'll merge dev into your branch and build again if you don't mind so that we're sure it build properly with the latest dev branch code.