Set<Shape> getTargetedFrom(Shape shape) {
            return findRelationshipsTo(shape).map(Relationship::getShape).collect(Collectors.toSet());
        }
Gets the set of shapes that refer to the given shape.

@param shape Shape to check for relationships to.
@return Returns the shapes that reference the given shape.
smithy-model/src/main/java/software/amazon/smithy/model/transform/MarkAndSweep.java