Include third party JS library into my drupal module
You still put them in the libraries folder (now you can also use a top-level /librariesfolder).

You need the libraries module, just like in 7.x. I'm also not 100% sure how well it works now in 8.x, in the JW Player module, I'm just using it to detect the location of the files: http://cgit.drupalcode.org/jw_player/tree/jw_player.module?h=8.x-1.x#n206