public boolean isSourceShape(ToShapeId shape) {
        return originalModel == null || isSource(originalModel.getShape(shape.toShapeId()).orElse(null));
    }
Checks if the given shape/ID is either not present in the original
model (thus a new, source shape), or is present and the filename of
the shape in the original model matches one of the defined
{@code sources}.

@param shape Shape or Shape ID to check.
@return Returns true if this shape is considered a source shape.
smithy-build/src/main/java/software/amazon/smithy/build/PluginContext.java