add per chunk index and index2

[sokra]

refactor index generation

What kind of change does this PR introduce?
feature, bugfix for MCEP

Did you add tests for your changes?
yes

Does this PR introduce a breaking change?
no

What needs to be documented once your changes are merged?
nothing

[webpack-bot]

For maintainers only:

• This need to be documented (issue in webpack/webpack.js.org will be filed when merged)

[alexander-akait]

@sokra can you provide link on issue?

[sokra]

webpack-contrib/mini-css-extract-plugin#115

[sokra]

see also webpack-contrib/mini-css-extract-plugin#130

[sokra]

After this PR MCEP can use the per chunk group index to get the module order:

https://github.com/webpack-contrib/mini-css-extract-plugin/blob/eff04a83ff7cdcbc88b282b42977bea05e782ba0/src/index.js#L383

There could still be cases where order can be done correctly (i. e. when extracting modules into a common chunk). It could make sense to emit a warning in these cases (but that probably annoy users since it doesn't matter in most cases).

[sokra]

cc @bawjensen

[alexander-akait]

@sokra i.e no additional work in MCEP?

[webpack-bot]

Thank you for your pull request! The most important CI builds succeeded, we’ll review the pull request soon.

[alexander-akait]

/cc @sokra #7603 (comment)

[kamilbrk]

Is there any reason why stats object is now different due to this PR? And, what's even more confusing, only when running webpack-serve, but not with standard one time build?

I'm using StatsWriterPlugin to generate a JSON file with entry points:
new StatsWriterPlugin({ fields: ['entrypoints'] })

This is what I get out of stats file with following versions:

• yarn add -D webpack@4.12.2 & npm run build:dev
• yarn add -D webpack@4.12.2 & npm run start (with webpack-serve@1.0.3)
• yarn add -D webpack@4.13.0 & npm run build:dev

{
  "entrypoints": {
    "bundle": {
      "chunks": [
        "vendors~bundle", "bundle"
      ],
      "assets": [
        "chunks/vendors~bundle.js", "bundle.js"
      ],
      "children": {},
      "childAssets": {}
    }
  }
}

And this is what I get now with this PR included, but only with webpack-serve:

• yarn add -D webpack@4.13.0 & npm run start (with webpack-serve@1.0.3)

{
  "entrypoints": {
    "bundle": {
      "chunks": [
        63, 64, 65, 66, 70,
        "vendors~bundle", "bundle"
      ],
      "assets": [
        "chunks/63.js", "chunks/64.js", "chunks/65.js", "chunks/66.js", "chunks/70.js",
        "chunks/vendors~bundle.js", "bundle.js"
      ],
      "children": {},
      "childAssets": {}
    }
  }
}

Turns out 63, 64, 65, 66 and 70 are parts of `chunks/vendors~bundle.js` chunk. It seems like webpack-serve is splitting that file into further separate chunks, for whatever reason, but only with webpack@4.13.0.