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/tlsdialer/saver_test.go

@@ -12,7 +12,6 @@ import (
 	"github.com/ooni/probe-cli/v3/internal/engine/netx/tlsdialer"
 	"github.com/ooni/probe-cli/v3/internal/engine/netx/trace"
 	"github.com/ooni/probe-cli/v3/internal/netxlite"
-	"github.com/ooni/probe-cli/v3/internal/netxlite/errorsx"
 )
 
 func TestSaverTLSHandshakerSuccessWithReadWrite(t *testing.T) {
@@ -71,7 +70,7 @@ func TestSaverTLSHandshakerSuccessWithReadWrite(t *testing.T) {
 			t.Fatal("unexpected NumBytes")
 		}
 		switch ev[idx].Name {
-		case errorsx.ReadOperation, errorsx.WriteOperation:
+		case netxlite.ReadOperation, netxlite.WriteOperation:
 		default:
 			t.Fatal("unexpected Name")
 		}