[BUG] Snowpack 3.1.1 fails with react-router-dom - incorrect namespace import

[BPScott]

Bug Report Quick Checklist

• I am on the latest version of Snowpack & all plugins.
• I use package manager npm (Fill in: npm, yarn, pnpm, etc).
• I run Snowpack on OS macos (Fill in: Windows, Mac, Linux, etc).
• I run Snowpack on Node.js v12+

Describe the bug

Using react-router-dom fails in Snowpack 3.1.1 when used in conjunction with
React 16.14.0. This previously worked in Snowpack 3.0.13.

Attempting to use the router results in the following error:

Uncaught TypeError: checkPropTypes is not a function
    at validatePropTypes (react.v16.14.0.js:1729)

I suspect this is because react's compiled output expects checkPropTypes to be a
function however it is imported using a namespace definition which is not callable.

// At the top of /_snowpack/pkg/react.v16.14.0.js
import * as checkPropTypes from '/_snowpack/pkg/prop-types.checkPropTypes.v15.7.2.js';

The issue goes away if I either:

• Use snowpack 3.0.13 and react 16.14.0 (downgrade snowpack, keep react version)
• Use snowpack 3.1.1 and react >=17.0.0 (keep snowpack version, update react)

To Reproduce

Reproduction repository is here

• clone repository and go into the react-router-prop-types folder
• npm install && npm start
• See error as described above

General steps

• Use snowpack 3.1.1
• Use react 16.14.0
• Use react-router-dom 5.2.0
• Add an importimport { BrowserRouter as Router } from "react-router-dom"; in App component
• Add the Router somewhere in your App component <Router></Router>

Expected behavior

Page to render, no errors

[FredKSchott]

@BPScott thanks for filing. Funny enough, I tracked this down to the snippet of code that we'd originally talked through related to ESM vs. CJS detection. More details here, including the fix to add back that check: #2934

I'll get this out tonight