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/legacy/netxlogger/netxlogger_test.go

@@ -10,7 +10,7 @@ import (
 	"github.com/apex/log/handlers/discard"
 	"github.com/ooni/probe-cli/v3/internal/engine/legacy/netx"
 	"github.com/ooni/probe-cli/v3/internal/engine/legacy/netx/modelx"
-	"github.com/ooni/probe-cli/v3/internal/netxlite/iox"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
 func TestGood(t *testing.T) {
@@ -33,7 +33,7 @@ func TestGood(t *testing.T) {
 		t.Fatal("expected non-nil resp here")
 	}
 	defer resp.Body.Close()
-	_, err = iox.ReadAllContext(context.Background(), resp.Body)
+	_, err = netxlite.ReadAllContext(context.Background(), resp.Body)
 	if err != nil {
 		t.Fatal(err)
 	}