folder.jpg album art is not imported

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

This was previously reported as https://github.com/elementary/music/issues/179. I believe that issue was incorrectly closed, because the patch applied fixes a different bug, not the one reported. (It was a fix to import album art embedded in the files, e.g. in id3. That's not the issue I'm having, and I don't think it was the issue this person was having either.)

## Describe the bug
All my albums are in individual folders in my library directory (like so: `Artist/Album/01 - Track Name.mp3`). Each folder has a folder.jpg or folder.png file. Music does not use the album art for any of these albums, the only album art shown is for albums where the art is embedded directly in the files.

### To Reproduce
1. Point Music to a folder with one album in it. The album should not have any embedded art, but should have a folder.jpg or folder.png file in the album folder.
2. Observe that the album art is not displayed.

### Expected behavior
I expect Music to find any reasonably named album art placed in the folder the album is in.

### Screenshots or screen recordings

## Platform Information
- OS: Arch Linux x86_64

Please check what applies:
- [x] I'm using the latest released stable version