Add "entrypoints" key to asset manifest

[samuelmeuli]

Problem:

We would like to generate the index.html file on the server. In order to get code splitting to work, we need to know which chunks are entrypoints of the React app (to be able to request these from the HTML file). Unfortunately, this currently doesn't seem to be possible (see #5225 (comment)).

Ideally, splitChunks.name would simply be set to true in the Webpack config, so the entrypoints could be inferred from the file names. However, this seems to lead to caching issues (see #5030 (comment)).

This PR implements @iansu's suggestion from #5513 (comment): It adds an additional "entrypoints" key to the asset-manifest.json file with the names of the files that need to be included in the generated HTML.

Before:

{
  "files": {
    "main.css": "/static/css/main.2cce8147.chunk.css",
    "main.js": "/static/js/main.fd729682.chunk.js",
    "main.js.map": "/static/js/main.fd729682.chunk.js.map",
    "runtime-main.js": "/static/js/runtime-main.f7c36e2a.js",
    "runtime-main.js.map": "/static/js/runtime-main.f7c36e2a.js.map",
    "static/js/2.ae086021.chunk.js": "/static/js/2.ae086021.chunk.js",
    "static/js/2.ae086021.chunk.js.map": "/static/js/2.ae086021.chunk.js.map",
    "index.html": "/index.html",
    "precache-manifest.be4e4fdd4a55417b6c618c280ec25a62.js": "/precache-manifest.be4e4fdd4a55417b6c618c280ec25a62.js",
    "service-worker.js": "/service-worker.js",
    "static/css/main.2cce8147.chunk.css.map": "/static/css/main.2cce8147.chunk.css.map",
    "static/media/logo.svg": "/static/media/logo.5d5d9eef.svg"
  }
}

After:

{
  "files": {
    "main.css": "/static/css/main.b100e6da.chunk.css",
    "main.js": "/static/js/main.70abf459.chunk.js",
    "main.js.map": "/static/js/main.70abf459.chunk.js.map",
    "runtime-main.js": "/static/js/runtime-main.fdecdd6f.js",
    "runtime-main.js.map": "/static/js/runtime-main.fdecdd6f.js.map",
    "static/js/2.a3ab821d.chunk.js": "/static/js/2.a3ab821d.chunk.js",
    "static/js/2.a3ab821d.chunk.js.map": "/static/js/2.a3ab821d.chunk.js.map",
    "index.html": "/index.html",
    "precache-manifest.47b41692c7614ae757144cfe8f04d3cb.js": "/precache-manifest.47b41692c7614ae757144cfe8f04d3cb.js",
    "service-worker.js": "/service-worker.js",
    "static/css/main.b100e6da.chunk.css.map": "/static/css/main.b100e6da.chunk.css.map",
    "static/media/logo.svg": "/static/media/logo.25bf045c.svg"
  },
  "entrypoints": [
    "static/js/runtime-main.fdecdd6f.js",
    "static/js/2.a3ab821d.chunk.js",
    "static/css/main.b100e6da.chunk.css",
    "static/js/main.70abf459.chunk.js"
  ]
}

Note: This PR is a draft because it requires shellscape/webpack-manifest-plugin#192 to be merged into webpack-manifest-plugin first. Without that change, the information about the order in which the assets need to be included isn't available yet (see #5513 (comment)).

Closes #5513

[iansu]

This is great. Thanks for putting this together. I think we can merge this as soon as the PR in webpack-manifest-plugin is merged.

[samuelmeuli]

The author has already merged it. I've updated the required package version.

[iansu]

Thanks!