cleanup: remove unnecessary legacy interfaces (#656)

This diff addresses another point of https://github.com/ooni/probe/issues/1956:

> - [ ] observe that we're still using a bunch of private interfaces for common interfaces such as the `Dialer`, so we can get rid of these private interfaces and always use the ones in `model`, which allows us to remove a bunch of legacy wrappers

Additional cleanups may still be possible. The more I cleanup, the more I see
there's extra legacy code we can dispose of (which seems good?).

internal/engine/netx/dialer/example_test.go

@@ -2,11 +2,11 @@ package dialer_test
 
 import (
 	"context"
-	"net"
 
 	"github.com/apex/log"
 	"github.com/ooni/probe-cli/v3/internal/engine/netx/dialer"
 	"github.com/ooni/probe-cli/v3/internal/engine/netx/trace"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
 func Example() {
@@ -16,7 +16,7 @@ func Example() {
 		DialSaver:      saver,
 		Logger:         log.Log,
 		ReadWriteSaver: saver,
-	}, &net.Resolver{})
+	}, netxlite.DefaultResolver)
 
 	ctx := context.Background()
 	conn, err := dlr.DialContext(ctx, "tcp", "8.8.8.8:53")