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/geoipx/geoipx_test.go

@@ -0,0 +1,59 @@
+package geoipx
+
+import (
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
+)
+
+const ipAddr = "8.8.8.8"
+
+func TestLookupASN(t *testing.T) {
+	t.Run("with valid IP address", func(t *testing.T) {
+		asn, org, err := LookupASN(ipAddr)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if asn != 15169 {
+			t.Fatal("unexpected ASN value", asn)
+		}
+		if org != "Google LLC" {
+			t.Fatal("unexpected org value", org)
+		}
+	})
+
+	t.Run("with invalid IP address", func(t *testing.T) {
+		asn, org, err := LookupASN("xxx")
+		if err == nil {
+			t.Fatal("expected an error here")
+		}
+		if asn != model.DefaultProbeASN {
+			t.Fatal("expected a zero ASN")
+		}
+		if org != model.DefaultProbeNetworkName {
+			t.Fatal("expected an empty org")
+		}
+	})
+}
+
+func TestLookupCC(t *testing.T) {
+	t.Run("with valid IP address", func(t *testing.T) {
+		cc, err := LookupCC(ipAddr)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if cc != "US" {
+			t.Fatal("invalid country code", cc)
+		}
+	})
+
+	t.Run("with invalid IP address", func(t *testing.T) {
+		cc, err := LookupCC("xxx")
+		if err == nil {
+			t.Fatal("expected an error here")
+		}
+		if cc != model.DefaultProbeCC {
+			t.Fatal("expected an empty cc")
+		}
+	})
+}