Generic Extractor should add short-hash of URL to filename

[d3fault]

Checklist

• I'm reporting a broken site support issue
• I've verified that I'm running youtube-dl version 2019.11.22
• 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 bug reports including closed ones
• I've read bugs section in FAQ

Verbose log

n/a

Description

Currently the generic extractor relies on the HTML Title and the last portion of the URL to generate a filename.

Ex:
youtube-dl https://www.w3.org/2010/05/video/mediaevents.html
gets you a file named:
HTML5 Video-mediaevents.mp4

This strategy could easily give filename collisions which result in an erroneous response of "this file has already been downloaded". The filename generated should contain a shorthash (first 8 characters of sha1) of the entire URL, or something similar.

Ex of what the filename should be:
HTML5 Video-mediaevents-fbea3c2c.mp4