Could not resolve js when building Angular library

The general task is to wrap up some third party UMD libraries in an Angular library and enhance them with strong typing.

I want to save the user of the library from having to manually add the UMD libraries in his project and include them by <script> tag for example.

Consider the following example project:

tsconfig.lib.json:

  "extends": "../../tsconfig.json",
  "compilerOptions": {
    "outDir": "../../out-tsc/lib",
    "allowJs": true,
    "declaration": true,
    "declarationMap": true,
    "inlineSources": true,
    "types": []
  },
  "exclude": [
    "src/test.ts",
    "**/*.spec.ts"
  ]
}

test-lib.service.ts:

import { Injectable } from '@angular/core';
import * as extLib from './external/externalLib';

@Injectable({
  providedIn: 'root'
})
export class TestLibService {
  private n: string = '';

  constructor() { }

  getStringFromNumber(n: number) {
    this.n = extLib.getString(n);
  }
}

When executing ng build I get the following error:

What am I missing?