Extract used prop types detection code

[alexzherdev]

There is a single spec failing here:

class Hello extends React.Component {
  render() {
    var {
      "aria-controls": ariaControls,
      propX,
      ...props } = this.props;
    return <div>Hello</div>;
  }
}
Hello.propTypes = {
  "aria-controls": PropTypes.string
};

The reason is because no-unused-prop-types ignores validation when it finds object spread:
https://github.com/yannickcr/eslint-plugin-react/blob/9258d796b2544fcee5980bca3b343e3dc8e7e3a2/lib/rules/no-unused-prop-types.js#L722-L725
Whereas prop-types doesn't:
https://github.com/yannickcr/eslint-plugin-react/blob/9258d796b2544fcee5980bca3b343e3dc8e7e3a2/lib/rules/prop-types.js#L702-L704

Not sure how to reconcile this.