refactor: merge dnsx and errorsx into netxlite (#517)

When preparing a tutorial for netxlite, I figured it is easier
to tell people "hey, this is the package you should use for all
low-level networking stuff" rather than introducing people to
a set of packages working together where some piece of functionality
is here and some other piece is there.

Part of https://github.com/ooni/probe/issues/1591

internal/engine/netx/dialer/bytecounter_test.go

@@ -9,7 +9,7 @@ import (
 	"testing"
 
 	"github.com/ooni/probe-cli/v3/internal/bytecounter"
-	"github.com/ooni/probe-cli/v3/internal/netxlite/iox"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 	"github.com/ooni/probe-cli/v3/internal/netxlite/mocks"
 )
 
@@ -27,7 +27,7 @@ func dorequest(ctx context.Context, url string) error {
 	if err != nil {
 		return err
 	}
-	if _, err := iox.CopyContext(ctx, io.Discard, resp.Body); err != nil {
+	if _, err := netxlite.CopyContext(ctx, io.Discard, resp.Body); err != nil {
 		return err
 	}
 	return resp.Body.Close()

internal/engine/netx/dialer/saver.go

@@ -6,7 +6,7 @@ import (
 	"time"
 
 	"github.com/ooni/probe-cli/v3/internal/engine/netx/trace"
-	"github.com/ooni/probe-cli/v3/internal/netxlite/errorsx"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
 // saverDialer saves events occurring during the dial
@@ -24,7 +24,7 @@ func (d *saverDialer) DialContext(ctx context.Context, network, address string)
 		Address:  address,
 		Duration: stop.Sub(start),
 		Err:      err,
-		Name:     errorsx.ConnectOperation,
+		Name:     netxlite.ConnectOperation,
 		Proto:    network,
 		Time:     stop,
 	})
@@ -61,7 +61,7 @@ func (c *saverConn) Read(p []byte) (int, error) {
 		Duration: stop.Sub(start),
 		Err:      err,
 		NumBytes: count,
-		Name:     errorsx.ReadOperation,
+		Name:     netxlite.ReadOperation,
 		Time:     stop,
 	})
 	return count, err
@@ -76,7 +76,7 @@ func (c *saverConn) Write(p []byte) (int, error) {
 		Duration: stop.Sub(start),
 		Err:      err,
 		NumBytes: count,
-		Name:     errorsx.WriteOperation,
+		Name:     netxlite.WriteOperation,
 		Time:     stop,
 	})
 	return count, err

internal/engine/netx/dialer/saver_test.go

@@ -9,7 +9,7 @@ import (
 	"time"
 
 	"github.com/ooni/probe-cli/v3/internal/engine/netx/trace"
-	"github.com/ooni/probe-cli/v3/internal/netxlite/errorsx"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 	"github.com/ooni/probe-cli/v3/internal/netxlite/mocks"
 )
 
@@ -44,7 +44,7 @@ func TestSaverDialerFailure(t *testing.T) {
 	if !errors.Is(ev[0].Err, expected) {
 		t.Fatal("unexpected Err")
 	}
-	if ev[0].Name != errorsx.ConnectOperation {
+	if ev[0].Name != netxlite.ConnectOperation {
 		t.Fatal("unexpected Name")
 	}
 	if ev[0].Proto != "tcp" {