refactor: spin geoipx off geolocate (#893)

A bunch of packages (including oohelperd) just need the ability to
use MaxMind-like databases. They don't need the additional functionality
implemented by the geolocate package. Such a package, in fact, is
mostly (if not only) needed by the engine package.

Therefore, move code to query MaxMind-like databases to a separate
package, and avoid depending on geolocate in all the packages for
which it's sufficient to use geoipx.

Part of https://github.com/ooni/probe/issues/2240

internal/engine/geolocate/cloudflare.go

@@ -23,7 +23,7 @@ func cloudflareIPLookup(
 		UserAgent:  model.HTTPHeaderUserAgent,
 	}).WithBodyLogging().Build().FetchResource(ctx, "/cdn-cgi/trace")
 	if err != nil {
-		return DefaultProbeIP, err
+		return model.DefaultProbeIP, err
 	}
 	r := regexp.MustCompile("(?:ip)=(.*)")
 	ip := strings.Trim(string(r.Find(data)), "ip=")