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/ptx/ptx.go

@@ -45,7 +45,7 @@ import (
 	"sync"
 
 	pt "git.torproject.org/pluggable-transports/goptlib.git"
-	"github.com/ooni/probe-cli/v3/internal/netxlite/iox"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
 // PTDialer is a generic pluggable transports dialer.
@@ -110,11 +110,11 @@ func (lst *Listener) forward(ctx context.Context, left, right net.Conn, done cha
 	wg.Add(2)
 	go func() {
 		defer wg.Done()
-		iox.CopyContext(ctx, left, right)
+		netxlite.CopyContext(ctx, left, right)
 	}()
 	go func() {
 		defer wg.Done()
-		iox.CopyContext(ctx, right, left)
+		netxlite.CopyContext(ctx, right, left)
 	}()
 	wg.Wait()
 }

internal/ptx/ptx_test.go

@@ -13,7 +13,7 @@ import (
 
 	"github.com/apex/log"
 	"github.com/ooni/probe-cli/v3/internal/atomicx"
-	"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"
 )
 
@@ -57,7 +57,7 @@ func TestListenerWorksWithFakeDialer(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	data, err := iox.ReadAllContext(context.Background(), resp.Body)
+	data, err := netxlite.ReadAllContext(context.Background(), resp.Body)
 	if err != nil {
 		t.Fatal(err)
 	}