`convert` plugin along with `copyartifacts` copies 14k garbage files into my music library :(

beets 2.5.1

I think the `convert` plugin puts the converted file in some non-empty directory and then `copy_artifacts` copies everything from there and puts a ton of garbage into my library.

```
$ fd '[0-9A-Fa-f]{8}-[0-9A-Fa-f]{4}-[0-9A-Fa-f]{4}-[0-9A-Fa-f]{4}-[0-9A-Fa-f]{12}' . | head -n20
./044B6978-46B2-4CAF-B079-C125A59499E3
./033EB6FD-DE05-41F5-8682-41599E05E519
./02E6C4B5-099E-4E27-A731-96A3FF748A35
./01DA927A-9345-412B-A953-3E3F2C62A30B
./071362CD-C76A-4365-AF12-875972AF156C
./07F1FF12-3DB0-47D6-9F9F-FAD8A55C9FBF
./0747EA60-F1A7-41EE-B400-2B5455B6EA8B
./03A9AAFD-1703-4841-8CA3-6977A440FB1C
./08281ECC-8B41-497A-ACFF-A80B35825C39
./048F5942-E5AD-4BAD-812D-DB634287281C
./06DFABA0-365F-46AC-993C-1B7F334BB48C
./031CAFC0-BD1E-4403-BC70-9960F0E957BF
./037B5E8A-1CA2-4BCD-8218-A937EBCB0C5B
./008BAB5B-4119-4B6E-9EA1-73CEEAC52C66
./04CF96A7-6BF1-47A1-9CA3-308B0C07A10B
./0593DB42-F49B-4355-8F22-38FED8ECC434
./0088BEA1-6343-444A-96D7-DE3C8C6E067E
./08306ADF-AE9A-4B59-93F5-D7AA562A5353
./010DBBBA-7098-472A-866B-CB683233CAE1
./05A19AE6-1FDA-4CC0-8B8E-18ECBB669F8A
$ fd '[0-9A-Fa-f]{8}-[0-9A-Fa-f]{4}-[0-9A-Fa-f]{4}-[0-9A-Fa-f]{4}-[0-9A-Fa-f]{12}' . | wc -l
14280
```

:((((((

Start by reproducing the issue with beets 2.5.1 using the convert and copyartifacts plugins, then trace how the converted file's directory is handed between them. Confirm the fix by ensuring copyartifacts does not copy the 14,280 UUID-named garbage files into the music library.

Written by the indexing model from the issue text.