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/model/measurement.go

@@ -1,20 +1,47 @@
 package model
 
-import (
-	"bytes"
-	"encoding/json"
-	"errors"
-	"net"
-	"time"
-)
-
 //
 // Definition of the result of a network measurement.
 //
 
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"net"
+	"time"
+)
+
 const (
+	// DefaultProbeASN is the default probe ASN as a number.
+	DefaultProbeASN uint = 0
+
+	// DefaultProbeCC is the default probe CC.
+	DefaultProbeCC = "ZZ"
+
 	// DefaultProbeIP is the default probe IP.
 	DefaultProbeIP = "127.0.0.1"
+
+	// DefaultProbeNetworkName is the default probe network name.
+	DefaultProbeNetworkName = ""
+
+	// DefaultResolverASN is the default resolver ASN.
+	DefaultResolverASN uint = 0
+
+	// DefaultResolverIP is the default resolver IP.
+	DefaultResolverIP = "127.0.0.2"
+
+	// DefaultResolverNetworkName is the default resolver network name.
+	DefaultResolverNetworkName = ""
+)
+
+var (
+	// DefaultProbeASNString is the default probe ASN as a string.
+	DefaultProbeASNString = fmt.Sprintf("AS%d", DefaultProbeASN)
+
+	// DefaultResolverASNString is the default resolver ASN as a string.
+	DefaultResolverASNString = fmt.Sprintf("AS%d", DefaultResolverASN)
 )
 
 // MeasurementTarget is the target of a OONI measurement.