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/experiment_test.go

@@ -26,7 +26,7 @@ func TestExperimentHonoursSharingDefaults(t *testing.T) {
 		name:         "probeIP",
 		locationInfo: &geolocate.Results{ProbeIP: "8.8.8.8"},
 		expect: func(m *model.Measurement) bool {
-			return m.ProbeIP == geolocate.DefaultProbeIP
+			return m.ProbeIP == model.DefaultProbeIP
 		},
 	}, {
 		name:         "probeASN",