Bug 1676038 - Change partial tab-to-search matching to avoid autofilling unexpected subdomains. r=adw

The previous approach was autofilling partial subdomains from search engines.
Having a search engine pointing to "developer.mozilla.org, when "moz" was typed
we ended up autofilling "moz[illa.org]" but pointing to "developer.mozilla.org".
Unfortunately that made impossible to actually go to "mozilla.org" by typing it.

The new approach instead allows tab-to-search results to appear even if there's
no autofill, because the provider checks autonomously the autofill threshold.
There's a downside though, with the old approach we could return more partial
matches, and the Muxer would pick one, depending on the autofilled host. For
example we could return "rover.ebay.com" and match it to "eb[ay.it]".
With the new approach we don't have an autofilled host, and fetching it would
be expensive, we can only compare the search string with the engine's host.
For this reason the patch also tries to partially match against the searchForm
host, that often points to a page the user visits more often.

Differential Revision: https://phabricator.services.mozilla.com/D96942