snowpack: [BUG] [3.1.2] symlinked path are not mounted anymore

Bug Report Quick Checklist

• I am on the latest version of Snowpack & all plugins.
• I use package manager pnpm
• I run Snowpack on OS Linux
• I run Snowpack on Node.js v12+ (v15)

Describe the bug

It seems mounted routes are not working anymore when they contain symlinks (on dev).

For instance, I used to mount a __ path, for which sub-path point to a firebase version (to mimic firebase reserved URLs on dev - https://firebase.google.com/docs/hosting/reserved-urls):

mount = {
 __: { url: '/__' }, // /__/firebase/${version} is symlinked to firebase node_modules
}

Referring to the mounted path was working in 3.0. It now returns 404:

 <script src="/__/firebase/8.2.9/firebase-app.js"></script>