No backgrounds shown with system backgrounds dir has subdirectories

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

I recalled https://github.com/elementary/switchboard-plug-pantheon-shell/issues/157 but that issue was fixed and there's been no `new_for_path` -> `new_for_uri`

## Describe the bug
This has been reported on NixOS https://github.com/NixOS/nixpkgs/issues/100293.
If there's subdirectories at the system backgrounds directory no wallpapers are loaded from there.

The layout of the system backgrounds directory would be like
```
/run/current-system/sw/share/backgrounds
nixos/
...bunch of elementary wallpapers
```

From the output there seems to be nothing of importance.

### To Reproduce
Steps to reproduce the behavior:
1. At your system background directory `$XDG_DATA_DIRS/share/backgrounds` have a subdirectory with pictures in it
2. Click on Wallpapers
3. Just a black background
4. Now delete that subdirectory and repeat. It's magic and now the wallpapers are there.

I actually did this with no local backgrounds btw.

### Expected behavior
It doesn't matter what directory structure at the backgrounds directory is, everything that is an image is loaded.

### Screenshots or screen recordings


## Platform Information

OS: NixOS
OS Version: NixOS 20.09 and 21.03-pre-git

- [ ] I'm using the latest version from git that I've manually compiled
- [x] I'm using the latest released stable version

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