importfeed: Avoid using youtube-dl when a feed does not contain an enclosure, but...

importfeed: Avoid using youtube-dl when a feed does not contain an enclosure, but only a link to an url which youtube-dl does not support

This is common in some feeds, which might mix some items with enclosures,
with others that link to posts or whatever. Before this, it would try to
use youtube-dl and fail, or if youtube-dl was not allowed, it would
incorrectly complain that an url was supported by youtube-dl.