[acast] 404 on various podcasts

[barsnick]

Checklist

• I'm reporting a broken site support
• I've verified that I'm running youtube-dl version 2020.12.31
• I've checked that all provided URLs are alive and playable in a browser
• I've checked that all URLs and arguments with special characters are properly quoted or escaped
• I've searched the bugtracker for similar issues including closed ones

Verbose log

[barsnick@paradise youtube-dl]$ python -m youtube_dl -v https://play.acast.com/s/forklart/ap-120479
[debug] System config: ['--prefer-free-formats']
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['-v', 'https://play.acast.com/s/forklart/ap-120479']
[debug] Encodings: locale UTF-8, fs utf-8, out utf-8, pref UTF-8
[debug] youtube-dl version 2020.12.31
[debug] Git HEAD: 40d66e07d
[debug] Python version 3.8.6 (CPython) - Linux-5.9.16-100.fc32.x86_64-x86_64-with-glibc2.2.5
[debug] exe versions: ffmpeg 4.2.4, ffprobe 4.2.4
[debug] Proxy map: {}
[acast] ap-120479: Downloading JSON metadata
[acast] ap-120479: Downloading JSON metadata
ERROR: Unable to download JSON metadata: HTTP Error 404: Not Found (caused by <HTTPError 404: 'Not Found'>); please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see  https://yt-dl.org/update  on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.
  File "/home/barsnick/Development/youtube-dl/youtube_dl/extractor/common.py", line 632, in _request_webpage
    return self._downloader.urlopen(url_or_request)
  File "/home/barsnick/Development/youtube-dl/youtube_dl/YoutubeDL.py", line 2248, in urlopen
    return self._opener.open(req, timeout=self._socket_timeout)
  File "/usr/lib64/python3.8/urllib/request.py", line 531, in open
    response = meth(req, response)
  File "/usr/lib64/python3.8/urllib/request.py", line 640, in http_response
    response = self.parent.error(
  File "/usr/lib64/python3.8/urllib/request.py", line 569, in error
    return self._call_chain(*args)
  File "/usr/lib64/python3.8/urllib/request.py", line 502, in _call_chain
    result = func(*args)
  File "/usr/lib64/python3.8/urllib/request.py", line 649, in http_error_default
    raise HTTPError(req.full_url, code, msg, hdrs, fp)

Description

I can find several podcasts on acast.com which cannot be downloaded with the existing extractor. It seems the acast site does not provide the utilized 'https://feeder.acast.com/api/v1/shows/%s/episodes/%s' % (channel, display_id) JSON file for these, or the display_id would need to be determined differently (not just from the URL, but from the webpage, which is currently not being loaded at all).

I have a fix which uses the webpage for determining the data, which works for previously working URLs as well as the one mentioned, and similar ones. I will post a PR.