cra-dev-webpack-modification.js

const rewire = require('rewire');
const defaults = rewire('react-scripts/scripts/start.js');
const configFactory = defaults.__get__('configFactory');
const devConfigFactory = configFactory('development');

// Do stuff
// i.e. devConfigFactory.module.rules.xxxxxx

const patchedDevConfigFactory = function() {
  return devConfigFactory;
};

defaults.__set__('configFactory', patchedDevConfigFactory);