`FileSystem.Mounted` doesn't mount library content

[badandbest]

Describe the bug

FileSystem.Mounted ignores files from libraries. This is really bad because it breaks every feature that relies on it.

To Reproduce

1. In the Code folder, create a file with the extension ".test".

2. Find all files in FileSystem.Mounted with the ".test" extension.

   foreach ( var testExtension in FileSystem.Mounted.FindFile( "", "*.test", true ) )
   {
       Log.Info( testExtension );
   }
   

3. FileSystem only mounts the file in the game project.

Expected behavior

Content in libraries needs to be mounted to FileSystem

Media/Files

Project.zip

Additional context

This is likely the root cause of https://github.com/Facepunch/sbox-issues/issues/5522 because scss files also do not get mounted.