[youtube] Extract duration in flat playlist mode

[Midorina]

Checklist

• I'm reporting a site feature request
• I've verified that I'm running youtube-dl version 2020.06.16.1
• I've searched the bugtracker for similar site feature requests including closed ones

Description

What the problem is:
Duration information of videos in a YouTube playlist are not given by YouTube-DL even though they're available, causing us to process each video (make an individual request for each video) to find out their duration. (which often leads to 429)

How it could be fixed:
Changing the video regex (maybe) and then parsing the duration of videos in the playlist extractor would fix this. (https://github.com/ytdl-org/youtube-dl/blob/master/youtube_dl/extractor/youtube.py#L332)

How your proposed solution would look like:
User calls extract_info() with process=False. The;

• Youtube ID
• Title
• and the duration of each video is given by YouTube-DL without having to pass process=True to extract_info() and make extra calls.