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/urlgetter/getter.go

@@ -10,7 +10,7 @@ import (
 	"github.com/ooni/probe-cli/v3/internal/engine/model"
 	"github.com/ooni/probe-cli/v3/internal/engine/netx/archival"
 	"github.com/ooni/probe-cli/v3/internal/engine/netx/trace"
-	"github.com/ooni/probe-cli/v3/internal/netxlite/errorsx"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 	"github.com/ooni/probe-cli/v3/internal/tunnel"
 )
 
@@ -57,7 +57,7 @@ func (g Getter) Get(ctx context.Context) (TestKeys, error) {
 	// hitting our httptransport that does error wrapping.
 	err = legacyerrorsx.SafeErrWrapperBuilder{
 		Error:     err,
-		Operation: errorsx.TopLevelOperation,
+		Operation: netxlite.TopLevelOperation,
 	}.MaybeBuild()
 	tk.FailedOperation = archival.NewFailedOperation(err)
 	tk.Failure = archival.NewFailure(err)