fix(webpack): production preload files

[clarkdo]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

In webpack production mode, some modules may be concatenated by scope hoisting via ModuleConcatenationPlugin, so these modules' mapping information will be lost.

For example: modules info for qux.js, quux.js and quuz.js are lost

{
  "modules": [
    {
      "chunks": [...],
      "identifier": "foo.js",
    },
    {
      "chunks": [...],
      "identifier": "bar.js"
    },
    {
      "chunks": [...],
      "identifier": "baz.js",
      "modules": [
        {
          "identifier": "qux.js"
        },
        {
          "identifier": "quux.js"
        }
        {
          "identifier": "quuz.js"
        }
      ]
    }
  ]
}

This pr change is traversing the ConcatenatedModule and populate the modules info.