Migrate to `symfony/dom-crawler`

[ash-jc-allen]

Up until now, the HttpDriver has used some regex and pattern-matching that I wrote myself to try and extract the links from the HTML. They've worked most of the time, but it's starting to become a bit more difficult to catch every use case.

This PR proposes requiring symfony/dom-crawler and using that to grab the favicons when using the HttpDriver. In theory, any devs using Favicon Fetcher shouldn't notice any difference.

I'll be checking this over the next day or two to make sure the migration will go as smoothly as possible 😄

[ash-jc-allen]

This PR also includes 2 small bug fixes that were spotted when doing this migration:

• If no favicons were found when using fetchAll, an exception wouldn't be thrown even if the throw flag had been used. This was because of a mistake in the condition that was checking for null, but the value would be an empty FaviconCollection. This should now be fixed.
• If no favicons were found when using fetchAll, we are supposed to attempt to find the default icon for the site. This wasn't being done, but should do now 🙂