how would I accomplish this same thing in webpack?

timezone.js

// if someone requires 'timezone', I want it to get this file
// and then based on some info in a global variable 'window.ENV'
// at runtime fetch a json file with a bunch of data specific to
// that timezone and then export the 'timezone' object, with
// that zone's data preloaded. in requireJS, I had a loader that
// looked something like this

// in timezone.js
define(['timezone_core'], function(timezone) {
  //see: http://requirejs.org/docs/plugins.html#apiload for the specifics of how this api works
  load: function (name, require, onload, config) {
    var usersTimezone = window.ENV.timezone
    // fetch the  data for the user's timezone async
    require(['timezone_data/' + usersTimezone], function (dataForUsersTimezone) {
      timezone.preloadWith(dataForUsersTimezone)
      // this is what we actually export to anyone that 'require's 'timezone'.
      // it will be the timezone object preloaded with the user's timezone data
      onload(timezone);
    });
  }
})
// when built, 'timezone_core' and this 'timezone' loader get concatenated into the
// main bundle, but it would make a seperate request to 'timezone_data/America_Denver'
// and wait for that to come back before providing the 'timezone' module to any
// other module that 'require's it. Note: I don't want all of the data for all 
// the timezones be concatenated into the bundle, that would make it huge. I just
// want it to to a request for the timezone it needs

// how would I do the same thing in webpack?