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/ubuntu_test.go

@@ -27,7 +27,7 @@ func TestUbuntuParseError(t *testing.T) {
 	if err == nil || !strings.HasPrefix(err.Error(), "XML syntax error") {
 		t.Fatalf("not the error we expected: %+v", err)
 	}
-	if ip != DefaultProbeIP {
+	if ip != model.DefaultProbeIP {
 		t.Fatalf("not the expected IP address: %s", ip)
 	}
 }