[node-resolve]: Support "browser" field external mappings

[guybedford]

• Rollup Plugin Name: node-resolve
• Rollup Plugin Version: latest

Feature Use Case

The package.json "browser" field supports external mappings of the form:

{
  "browser" {
    "external": "newexternal",
    "./local.js": "external/subpath"
  }
}

For example see https://github.com/sodium-friends/sodium-universal/blob/master/package.json#L14

Feature Proposal

By default the node-resolve plugin is currently handling these as local package paths instead of external resolutions as they should be.

[volkanunsal]

This is very much needed.

[shellscape]

@volkanunsal we'd happily review a pull request that fulfilled this request

[stale]

Hey folks. This issue hasn't received any traction for 60 days, so we're going to close this for housekeeping. If this is still an ongoing issue, please do consider contributing a Pull Request to resolve it. Further discussion is always welcome even with the issue closed. If anything actionable is posted in the comments, we'll consider reopening it. ⓘ

[guybedford]

I'm going to reopen this one as it seems like an important omission in the default resolution behaviour unless I've missed that it's been addressed already.

[volkanunsal]

I looked at the source code to potentially provide a patch for it, but the resolution algorithm is so convoluted and written in an idiosyncratic way. I can't make head or tail of it.