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/hirl/hirl.go

@@ -14,7 +14,7 @@ import (
 	"github.com/ooni/probe-cli/v3/internal/engine/model"
 	"github.com/ooni/probe-cli/v3/internal/engine/netx"
 	"github.com/ooni/probe-cli/v3/internal/engine/netx/archival"
-	"github.com/ooni/probe-cli/v3/internal/netxlite/errorsx"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 	"github.com/ooni/probe-cli/v3/internal/randx"
 )
 
@@ -293,7 +293,7 @@ func RunMethod(ctx context.Context, config RunMethodConfig) {
 		count, err := conn.Read(data)
 		if err != nil {
 			// We expect this method to terminate w/ timeout
-			if err.Error() == errorsx.FailureGenericTimeoutError {
+			if err.Error() == netxlite.FailureGenericTimeoutError {
 				err = nil
 			}
 			result.Err = err

internal/engine/experiment/hirl/hirl_test.go

@@ -12,7 +12,7 @@ import (
 	"github.com/ooni/probe-cli/v3/internal/engine/model"
 	"github.com/ooni/probe-cli/v3/internal/engine/netx"
 	"github.com/ooni/probe-cli/v3/internal/engine/netx/archival"
-	"github.com/ooni/probe-cli/v3/internal/netxlite/errorsx"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
 func TestNewExperimentMeasurer(t *testing.T) {
@@ -97,7 +97,7 @@ func TestCancelledContext(t *testing.T) {
 		t.Fatal("unexpected FailureList length")
 	}
 	for _, failure := range tk.FailureList {
-		if *failure != errorsx.FailureInterrupted {
+		if *failure != netxlite.FailureInterrupted {
 			t.Fatal("unexpected failure")
 		}
 	}