rollup-plugin-postcss: ESM library generated with rollup-plugin-postcss throws Cannot find module '../node_modules/style-inject/dist/style-inject.es.js'

Apologies if this is more of a usage question than a bug, I have posted here at Stackoverflow.

We are maintaining an internal library which is exporting ESM modules using Rollup. We have just recently switched to using CSS modules, which we have set with rollup-plugin-postcss. We want to inject these styles into the head rather than have an external file.

Our built bundle generates the ESM file with:

import styleInject from '../node_modules/style-inject/dist/style-inject.es.js';

Our consuming library then fails with

 Uncaught Error: Cannot find module '../node_modules/style-inject/dist/style-inject.es.js'

I would expect the ESM export to import styleInject from 'style-inject' and style-inject to be included in the package-lock.json as a dependency. Even manually adding the dependency as part of our library which then includes it in the consuming app doesn’t allow it to resolve. What is the correct way of using CSS Modules and injecting into the head for the consumer of a library?

rollup.config.js

import resolve from '@rollup/plugin-node-resolve';
import commonjs from '@rollup/plugin-commonjs';
import babel from '@rollup/plugin-babel';
import json from '@rollup/plugin-json';
import postcss from 'rollup-plugin-postcss';
import pkg from './package.json';
import fg from 'fast-glob';
import path from 'path';

export default [
  {
    input: 'src/index.js',
    external: external(),
    output: [
      {
        name: '@my/packageName',
        file: pkg.module,
        format: 'es',
        sourcemap: true,
      },
    ],
    plugins: [
      {
        name: 'watch-external',
        async buildStart() {
          const files = await fg(['src/index.d.ts', 'playground/**/*']);
          for (let file of files) {
            this.addWatchFile(path.resolve(file));
          }
        },
      },
      json(),
      postcss({
        modules: true,
      }),
      babel({
        exclude: /node_modules/,
        babelHelpers: 'runtime',
        babelrc: false,
        presets: [
          [
            '@babel/preset-env',
            {
              modules: false,
              useBuiltIns: 'entry',
              corejs: 3,
              targets: {
                ie: 11,
              },
            },
          ],
          '@babel/preset-react',
        ],
        plugins: [
          '@babel/plugin-transform-runtime',
          '@babel/plugin-proposal-class-properties',
          '@babel/plugin-proposal-export-namespace-from',
        ],
      }),
      commonjs(),
    ],
  },
];

function external() {
  const { dependencies = {}, peerDependencies = {} } = pkg;

  const externals = [
    ...Object.keys(dependencies),
    ...Object.keys(peerDependencies),
  ];

  return id =>
    // match 'lodash' and 'lodash/fp/isEqual' for example
    externals.some(dep => id === dep || id.startsWith(`${dep}/`));
}