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/geolocate/fake_test.go

@@ -4,7 +4,7 @@ import (
 	"net/http"
 	"time"
 
-	"github.com/ooni/probe-cli/v3/internal/netxlite/iox"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
 type FakeTransport struct {
@@ -15,7 +15,7 @@ type FakeTransport struct {
 func (txp FakeTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 	time.Sleep(10 * time.Microsecond)
 	if req.Body != nil {
-		iox.ReadAllContext(req.Context(), req.Body)
+		netxlite.ReadAllContext(req.Context(), req.Body)
 		req.Body.Close()
 	}
 	if txp.Err != nil {