Bug report

What is the current behavior?

When the output.path is set to /, webpack 5 will fail with an EISDIR error. It works in webpack 4.
/ is commonly used when targeting an in-memory filesystem such as https://github.com/streamich/memfs; we used it quite often in tests.

If the current behavior is a bug, please provide the steps to reproduce.

git clone https://github.com/sodatea/webpack-rootdir-bug.git
cd webpack-rootdir-bug
yarn && yarn build

What is the expected behavior?

Should successfully build.

Other relevant information:
webpack version: v5.0.0-beta.14
Node.js version: v12.16.1
Operating System: macOS Catalina 10.15.3
Additional tools: None