playlist: sorting of an on-disk playlist is ignored

When using the playlist plugin for querying (syntax `playlist:/path/to/list.m3u`) it seems like the sorting of the list is lost.

### Problem

We see that even though the query sorts by bpm and the playlist file (saved by smartplaylist plugin earlier) is sorted by bpm ascending, it gets lost when the `playlist:....` syntax is used:

```

prod  (beets)    ~/git/beets   all_in ●  beet config | grep include:
include: [smart_playlists.yaml]
prod  (beets)    ~/git/beets   all_in ●  cat $BEETSDIR/smart_playlists.yaml
smartplaylist:
auto: yes
playlists:
- name: 110..130_TechnoJungleDub_Hi.m3u
query: >
year:1993.. bpm:110..130 "genre::(?i)Jungle|Footwork|Drum & Bass|Drum and Bass|Dubstep|Bass" initial_key+ bpm+ bitrate:200000..

- name: mood_happy_relaxed.m3u
query: >
mood_happy:0.8.. mood_relaxed:0.8.. bpm+ initial_key+

- name: mood_sad_relaxed.m3u
query: >
mood_sad:0.9.. mood_relaxed:0.9.. bpm+ initial_key+

- name: mood_acoustic_relaxed.m3u
query: >
mood_acoustic:0.9.. mood_relaxed:0.9.. bpm+ initial_key+

  prod  (beets)    ~/git/beets   all_in ●  beet ls mood_acoustic:0.9.. mood_relaxed:0.9.. bpm+ initial_key+ -f '$bpm, $initial_key, $artist, $title' | head -5
70, Dm, Aphex Twin, Nanou 2
83, , µ-Ziq, Fall Of Antioch
90, F, Wisp, Around Corners
91, A, Portishead, Deep Water
92, G, Clueso, Barfuß

prod  (beets)    ~/git/beets   all_in ●  beet ls playlist:/remote/data/music-library/0-playlists/mood_acoustic_relaxed.m3u -f '$bpm, $initial_key, $artist, $title' | head -5
148, , Long Beach Dub Allstars, Love Her Madly
166, D, Deep & Wide, Seven Seas
161, Fm, Squarepusher, 213 (Maritime Eposis)
105, Em, Squarepusher, Spymania Theme
108, C, Harold Budd, Balthus Bemused by Colour

  prod  (beets)    ~/git/beets   all_in ●  head -5 /remote/data/music-library/0-playlists/mood_acoustic_relaxed.m3u
/remote/data/music-library/IDM,Electronica/Aphex Twin/2001 - Aphex Twin - Drukqs/30 - Aphex Twin - Drukqs - Nanou 2 (2001).mp3
/remote/data/music-library/IDM,Electronica/u-Ziq/2003 - u-Ziq - Bilious Paths/11 - u-Ziq - Bilious Paths - Fall Of Antioch (2003).mp3
/remote/data/music-library/IDM,Electronica/Wisp/2004 - Wisp - About Things That Never Were/09 - Wisp - About Things That Never Were - Around Corners (2004).mp3
/remote/data/music-library/DowntempoTriphopDubAcidjazz/Portishead/2008 - Portishead - Third/07 - Portishead - Third - Deep Water (2008).m4a
/remote/data/music-library/AlternativeRockPop/Clueso/2008 - Clueso - So sehr dabei/01 - Clueso - So sehr dabei - Barfuss (2008).mp3
```

### Setup

* OS: Manjaro
* Python version: 3.9
* beets version: git latest
* Turning off plugins made problem go away (yes/no): FIXME

My configuration (output of `beet config`) is:

See above for the IMHO relevant parts. Can certainly provide more on demand.