[youtube] Support channel search

[pukkandan]

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Code from yt-dlp/yt-dlp@cd68417
Closes #29071

[dirkf]

In addition to this it seems that we could have resurrected YoutubeSearchURLIE with a class body as below:

    # override SearchInfoExtractor.suitable() without copy/paste - surely there must be a better way?
    suitable = classmethod(InfoExtractor.suitable.__func__)

    def _real_extract(self, url):
        qs = parse_qs(url)
        query = (qs.get('search_query') or qs.get('q'))[-1]
        self._SEARCH_PARAMS = qs.get('sp', (None, ))[-1]
        return super(YoutubeSearchURLIE, self)._real_extract('ytsearchall:%s' % query)

But your #27749 is more satisfactory.

[pukkandan]

That was basically my original implementation. Except I didn't use super calls. e309ff3