BUG: Incorrect URLs for npm packages, causing 404 in watch mode

Quick checklist

• I am using the latest version of Snowpack and all plugins.

What package manager are you using?

npm

What operating system are you using?

Windows

Describe the bug

Some URLs generated by Snowpack in --watch mode are 404s. Partly, the reason for that is non-relative URLs, addressed in #3597. However, the incorrect URLs point to files that do not exist altogether.

Steps to reproduce

1. git clone https://github.com/hdodov/repro-snowpack-url-issue
2. cd repro-snowpack-url-issue
3. npm i
4. npx snowpack build --watch
5. Output build folder should contain incorrect paths.

Link to minimal reproducible example (optional)

https://github.com/hdodov/repro-snowpack-url-issue