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/resolver/saver.go

@@ -5,11 +5,12 @@ import (
 	"time"
 
 	"github.com/ooni/probe-cli/v3/internal/engine/netx/trace"
+	"github.com/ooni/probe-cli/v3/internal/model"
 )
 
 // SaverResolver is a resolver that saves events
 type SaverResolver struct {
-	Resolver
+	model.Resolver
 	Saver *trace.Saver
 }
 
@@ -69,5 +70,5 @@ func (txp SaverDNSTransport) RoundTrip(ctx context.Context, query []byte) ([]byt
 	return reply, err
 }
 
-var _ Resolver = SaverResolver{}
+var _ model.Resolver = SaverResolver{}
 var _ RoundTripper = SaverDNSTransport{}