fullcalendar: Jest test runner cannot find fullcalendar modules

Bug

See this repository for reproduction steps: https://github.com/sgoodrow/test-fullcalendar-v5

Clone and run:

1. yarn install
2. yarn test

FAIL  src/App.test.js
  ● Test suite failed to run

    Cannot find module '@fullcalendar/react' from 'DemoApp.jsx'

    However, Jest was able to find:
    	'./DemoApp.jsx'

    You might want to include a file extension in your import, or update your 'moduleFileExtensions', which is currently ['web.js', 'js', 'web.ts', 'ts', 'web.tsx', 'tsx', 'json', 'web.jsx', 'jsx', 'node'].

    See https://jestjs.io/docs/en/configuration#modulefileextensions-array-string

    However, Jest was able to find:
    	'./App.css'
    	'./App.js'
    	'./App.test.js'

    You might want to include a file extension in your import, or update your 'moduleFileExtensions', which is currently ['web.js', 'js', 'web.ts', 'ts', 'web.tsx', 'tsx', 'json', 'web.jsx', 'jsx', 'node'].

    See https://jestjs.io/docs/en/configuration#modulefileextensions-array-string

      1 | import React from "react";
    > 2 | import FullCalendar from "@fullcalendar/react";
        | ^
      3 | import dayGridPlugin from "@fullcalendar/daygrid";
      4 |
      5 | export default class DemoApp extends React.Component {

      at Resolver.resolveModule (node_modules/jest-resolve/build/index.js:259:17)
      at Object.<anonymous> (src/DemoApp.jsx:2:1)

Test Suites: 1 failed, 1 total
Tests:       0 total
Snapshots:   0 total
Time:        1.985s

This repository was created by:

1. Running create-react-app
2. Installing the v5 react fullcalendar per these instructions

React/Vue/Angular/etc

I happen to be using React, but the import error occurs for the plugin files, too, so this is a broader issue than just the React connector. AKA, if you change the import order:

import React from "react";
import dayGridPlugin from "@fullcalendar/daygrid";
import FullCalendar from "@fullcalendar/react";

You will see Jest get hung up on daygrid instead:

 FAIL  src/App.test.js
  ● Test suite failed to run

    Cannot find module '@fullcalendar/daygrid' from 'DemoApp.jsx'

    However, Jest was able to find:
    	'./DemoApp.jsx'

Note also that this does not occur with 4.x.