@loadable/babel-plugin doesn't work with declared functions

🐛 Bug Report

I have a case where I need to pass a loader function to a @loadable/component and the @loadable/babel-plugin do not work as expected. The function passed as an argument, won’t be capture by the babel plugin, making the InnerComponent fail on SSR with a

SSR requires @loadable/babel, please install it

To Reproduce

Running the following will produce the above error when trying to render on server-side:

// react-router v4 route.js file

const loadBuses = () =>
  import(/* webpackChunkName: "buses" */ './components/BusesContainer/BusesContainer.page');

export default [
  {
    path: '/',
    exact: true,
    component: asyncComponent({
      loader: loadBuses
    })
  }];

// asyncComponent.js
export function asyncComponent({ loader }) {
  const LoadableComponent = loadable(loader);

  const AsyncRouteComponent = props => {
    return <LoadableComponent {...props} />;
  };

  return AsyncRouteComponent;
}

After running babel with the @loadable/babel-plugin outputs:

var loadBuses = function loadBuses() {
  return import(
  /* webpackChunkName: "buses" */
  './components/BusesContainer/BusesContainer.page');
};

Expected behavior

I’ll expect that the @loadable/babel-plugin transpiles the loader function with the right methods so It can be used by @loadable/component:

var loadBuses = function loadBuses() {
  return {
      chunkName: function chunkName() {
        return "buses";
      },
      isReady: function isReady(props) {
        if (typeof __webpack_modules__ !== 'undefined') {
          return !!__webpack_modules__[this.resolve(props)];
        }

        return false;
      },
      requireAsync: function requireAsync() {
        return import(
        /* webpackChunkName: "buses" */
        './components/BusesContainer/BusesContainer.page');
      },
      requireSync: function requireSync(props) {
        var id = this.resolve(props);

        if (typeof __webpack_require__ !== 'undefined') {
          return __webpack_require__(id);
        }

        return eval('module.require')(id);
      },
      resolve: function resolve() {
        if (require.resolveWeak) {
          return require.resolveWeak("./components/BusesContainer/BusesContainer.page");
        }

        return require('path').resolve(__dirname, "./components/BusesContainer/BusesContainer.page");
      }
    }

Run npx envinfo --system --binaries --npmPackages @loadable/component,@loadable/server,@loadable/webpack-plugin,@loadable/babel-plugin --markdown --clipboard

## System:
 - OS: macOS 10.14.1
 - CPU: (8) x64 Intel(R) Core(TM) i7-6820HQ CPU @ 2.70GHz
 - Memory: 555.12 MB / 16.00 GB
 - Shell: 5.3 - /bin/zsh
## Binaries:
 - Node: 8.10.0 - /usr/local/bin/node
 - Yarn: 1.12.3 - /usr/local/bin/yarn
 - npm: 6.4.1 - ~/.npm-packages/bin/npm
## npmPackages:
 - @loadable/babel-plugin: ^5.2.2 => 5.2.2
 - @loadable/component: ^5.2.2 => 5.2.2
 - @loadable/server: ^5.2.2 => 5.2.2
 - @loadable/webpack-plugin: ^5.2.2 => 5.2.2

[gregberge]

Done in #192

[lfantone]

Amazing!! 🎊