Elm module groups resolver

.parcelrc

{
  "extends": "@parcel/config-default",
  "resolvers": ["parcel-resolver-elm", "..."]
}

example.js

import {Elm} from 'elm:widget-a';

console.log(Elm);

package.json

{
  "name": "elmexample",
  "version": "0.0.0",
  "devDependencies": {
    "@babel/core": "^7.0.0",
    "@parcel/babel-register": "2.6.0",
    "parcel-resolver-elm": "*",
    "parcel": "2.6.0"
  },
  "parcel-resolver-elm": {
    "widget-a": [
      "./src/Main.elm",
      "./src/MainB.elm",
      "./src/MainC.elm"
    ]
  }
}

resolver.js

// @flow
const {Resolver} = require('@parcel/plugin');
const {loadConfig} = require('@parcel/utils');
const {md} = require('@parcel/diagnostic');
const nullthrows = require('nullthrows');
const path = require('path');

module.exports = (new Resolver({
  async resolve({dependency, options, specifier}) {
    let parent = dependency.sourcePath;
    if (specifier.startsWith('elm:') && parent != null) {
      let name = specifier.slice('elm:'.length);

      let pkg = await loadConfig(
        options.inputFS,
        parent,
        ['package.json'],
        options.projectRoot,
      );

      let invalidateOnFileChange = pkg?.files.map(f => f.filePath) ?? [];
      let invalidateOnFileCreate = [
        {fileName: 'package.json', aboveFilePath: parent},
      ];

      let modules = pkg?.config['parcel-resolver-elm'];
      if (modules) {
        let pkgFile = nullthrows(pkg?.files)[0].filePath;
        let pkgDir = path.dirname(pkgFile);
        let mod = modules[name];
        if (mod != null && mod.length > 0) {
          let mainPath = path.resolve(pkgDir, mod[0]);
          let mainDir = path.dirname(mainPath);

          let newSpecifier = `${mod[0]}?${mod
            .slice(1)
            .map(m => path.relative(mainDir, path.resolve(pkgDir, m)))
            .join('&')}`;

          return {
            invalidateOnFileChange,
            invalidateOnFileCreate,
            filePath: path.join(pkgDir, 'index.js'),
            code: `export * from "${newSpecifier}";`,
          };
        } else {
          return {
            invalidateOnFileChange,
            invalidateOnFileCreate,
            diagnostics: {
              message: md`No module defined with name "${name}" in ${pkgFile}`,
            },
          };
        }
      }

      return {
        invalidateOnFileChange,
        invalidateOnFileCreate,
      };
    }
  },
}) /*: Resolver */);