As said before, I would suggest not to rely on such a resource in Sweet Home 3D (they can change like icons recently), and would copy [font=courier new]lightSource.obj[/font] in your plug-in.
You could even use your own shape naming it in the OBJ file with a prefix equal to [font=courier new]ModelManager.LIGHT_SHAPE_PREFIX[/font] to ensure it has the same behavior as [font=courier new]lightSource.obj[/font].