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/netxlite/quirks_test.go

@@ -5,7 +5,6 @@ import (
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
-	"github.com/ooni/probe-cli/v3/internal/netxlite/errorsx"
 )
 
 func TestQuirkReduceErrors(t *testing.T) {
@@ -35,19 +34,19 @@ func TestQuirkReduceErrors(t *testing.T) {
 
 	t.Run("multiple errors with meaningful ones", func(t *testing.T) {
 		err1 := errors.New("mocked error #1")
-		err2 := errorsx.NewErrWrapper(
-			errorsx.ClassifyGenericError,
-			errorsx.CloseOperation,
+		err2 := NewErrWrapper(
+			ClassifyGenericError,
+			CloseOperation,
 			errors.New("antani"),
 		)
-		err3 := errorsx.NewErrWrapper(
-			errorsx.ClassifyGenericError,
-			errorsx.CloseOperation,
-			errorsx.ECONNREFUSED,
+		err3 := NewErrWrapper(
+			ClassifyGenericError,
+			CloseOperation,
+			ECONNREFUSED,
 		)
 		err4 := errors.New("mocked error #3")
 		result := quirkReduceErrors([]error{err1, err2, err3, err4})
-		if result.Error() != errorsx.FailureConnectionRefused {
+		if result.Error() != FailureConnectionRefused {
 			t.Fatal("wrong result")
 		}
 	})