Parcel will duplicate modules across entry points

[jakearchibald]

🐛 bug report

https://github.com/GoogleChromeLabs/tooling.report/tree/4159c1e7a96cd5052e86aeafc1ff214f556074a0/tests/output-module-formats/subtests/es-modules/parcel

In this example two entry points import a common module, and Parcel duplicates the module in each entry point, leading to out of sync state.

It seems like this was fixed in particular cases (#4302), but this example is a library (isLibrary is true), so there's no way to know if each entry point will be in its own realm or not.

I think the behaviour needs to change so Parcel only duplicates modules if it's sure that they stay in different realms, as in the entry points are HTML. Although, there may be cases where the developer still wants to opt out of this.

[jakearchibald]

In the above test, if I change the engines to:

"engines": {
  "node": ">=12.x"
}

I still get duplication, and also some browser-specific polyfills which seems weird.

[mischnic]

I feel like we need to have a different bundling strategy (in which other bundles can be loaded synchronously as in ESM) based on the output format (cjs & esm). (So as though isEntry is false, I think)