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/fsx/example_test.go

@@ -9,7 +9,7 @@ import (
 	"syscall"
 
 	"github.com/ooni/probe-cli/v3/internal/fsx"
-	"github.com/ooni/probe-cli/v3/internal/netxlite/iox"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
 func ExampleOpenFile_openingDir() {
@@ -27,7 +27,7 @@ func ExampleOpenFile_openingFile() {
 	if err != nil {
 		log.Fatal("unexpected error", err)
 	}
-	data, err := iox.ReadAllContext(context.Background(), filep)
+	data, err := netxlite.ReadAllContext(context.Background(), filep)
 	if err != nil {
 		log.Fatal("unexpected error", err)
 	}