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/fake_test.go

@@ -6,7 +6,7 @@ import (
 	"net/http"
 	"time"
 
-	"github.com/ooni/probe-cli/v3/internal/netxlite/iox"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
 type FakeDialer struct {
@@ -31,7 +31,7 @@ func (txp FakeTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 		return txp.Func(req)
 	}
 	if req.Body != nil {
-		iox.ReadAllContext(req.Context(), req.Body)
+		netxlite.ReadAllContext(req.Context(), req.Body)
 		req.Body.Close()
 	}
 	if txp.Err != nil {