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/websteps/control.go

@@ -6,7 +6,7 @@ import (
 	"github.com/ooni/probe-cli/v3/internal/engine/httpx"
 	errorsxlegacy "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"
 )
 
 // Control performs the control request and returns the response.
@@ -21,7 +21,7 @@ func Control(
 	// make sure error is wrapped
 	err = errorsxlegacy.SafeErrWrapperBuilder{
 		Error:     clnt.PostJSON(ctx, resourcePath, creq, &out),
-		Operation: errorsx.TopLevelOperation,
+		Operation: netxlite.TopLevelOperation,
 	}.MaybeBuild()
 	return
 }