[cleanup] Refactor `__init__.py` (#2570)

* Split `__init__` code into multiple functions
* Clean up validation code by grouping similar types of options
* Expose `parse_options` to third parties

yt_dlp/options.py

@@ -1182,7 +1182,7 @@ def create_parser():
         help='Do not write video description (default)')
     filesystem.add_option(
         '--write-info-json',
-        action='store_true', dest='writeinfojson', default=False,
+        action='store_true', dest='writeinfojson', default=None,
         help='Write video metadata to a .info.json file (this may contain personal information)')
     filesystem.add_option(
         '--no-write-info-json',

yt_dlp/postprocessor/ffmpeg.py

@@ -404,7 +404,7 @@ class FFmpegPostProcessor(PostProcessor):
 
 class FFmpegExtractAudioPP(FFmpegPostProcessor):
     COMMON_AUDIO_EXTS = ('wav', 'flac', 'm4a', 'aiff', 'mp3', 'ogg', 'mka', 'opus', 'wma')
-    SUPPORTED_EXTS = ('best', 'aac', 'flac', 'mp3', 'm4a', 'opus', 'vorbis', 'wav', 'alac')
+    SUPPORTED_EXTS = ('aac', 'flac', 'mp3', 'm4a', 'opus', 'vorbis', 'wav', 'alac')
 
     def __init__(self, downloader=None, preferredcodec=None, preferredquality=None, nopostoverwrites=False):
         FFmpegPostProcessor.__init__(self, downloader)

yt_dlp/utils.py

@@ -3603,6 +3603,9 @@ def match_str(filter_str, dct, incomplete=False):
 
 
 def match_filter_func(filter_str):
+    if filter_str is None:
+        return None
+
     def _match_func(info_dict, *args, **kwargs):
         if match_str(filter_str, info_dict, *args, **kwargs):
             return None