default Collection<? extends Shape> getStartingShapes(Model model) {
        return model.toSet();
    }
Returns the set of shapes to pump through the selector.

<p>This method returns all shapes in the model by default. Some selectors can return a subset of shapes if
the selector can filter shapes more efficiently. For example, when selecting "structure", it is far less work
to leverage {@link Model#toSet(Class)} than it is to send every shape
through every selector.

@return Returns the starting shapes to push through the selector.
smithy-model/src/main/java/software/amazon/smithy/model/selector/InternalSelector.java