umi: [Bug] mfsu breaks source map

What happens?

There are warning about source maps when turn on mfsu. And lots of node_modules packages cannot be mapped.

DevTools failed to load source map: Could not parse content for http://localhost:8000/react-router.js.map: Unexpected token < in JSON at position 0
DevTools failed to load source map: Could not parse content for http://localhost:8000/index.js.map: Unexpected token < in JSON at position 0
DevTools failed to load source map: Could not parse content for http://localhost:8000/react-router-config.js.map: Unexpected token < in JSON at position 0

Mini Showcase Repository(REQUIRED)

Please provide a repository using yarn create @umijs/umi-app then upload to your GitHub 请提供一个可复现的仓库，使用 yarn create @umijs/umi-app 创建，并上传到你的 GitHub 仓库

https://github.com/cagen/umi-test

How To Reproduce

Steps to reproduce the behavior:

1. npx @umijs/create-umi-app
2. add mfsu: {} in .umirc.ts
3. npm start
4. open chrome devtools to see the warnings

Expected behavior There should not be any source maps warnings, and node_modules packages can be mapped correctly.

Context

• Umi Version:3.5.18
• Node Version:12.22.1
• Platform:macOS