MAGN-4051 : Add migration for PolyCurve.Fillet

This PR has two components. The first is simply updating the ProtoGeometry.Migrations.xml file to include the name change from PolyCurve.FilletPlanarPolyCurve to PolyCurve.Fillet, upgrading the node names in old graphs.

The second changes the way Dynamo searches for the ...Migrations.xml file to be able to locate the file more reliable when inside of Revit.

@ikeough PTAL