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/experiment/webconnectivity/control.go

@@ -7,7 +7,7 @@ import (
 	"github.com/ooni/probe-cli/v3/internal/engine/httpx"
 	legacyerrorsx "github.com/ooni/probe-cli/v3/internal/engine/legacy/errorsx"
 	"github.com/ooni/probe-cli/v3/internal/engine/model"
-	"github.com/ooni/probe-cli/v3/internal/netxlite/errorsx"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
 // ControlRequest is the request that we send to the control
@@ -62,7 +62,7 @@ func Control(
 	// make sure error is wrapped
 	err = legacyerrorsx.SafeErrWrapperBuilder{
 		Error:     clnt.PostJSON(ctx, "/", creq, &out),
-		Operation: errorsx.TopLevelOperation,
+		Operation: netxlite.TopLevelOperation,
 	}.MaybeBuild()
 	sess.Logger().Infof("control for %s... %+v", creq.HTTPRequest, err)
 	(&out.DNS).FillASNs(sess)