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/hhfm/fake_test.go

@@ -6,7 +6,7 @@ import (
 	"net/http"
 	"time"
 
-	"github.com/ooni/probe-cli/v3/internal/netxlite/iox"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
 type FakeDialer struct {
@@ -31,7 +31,7 @@ func (txp FakeTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 		return txp.Func(req)
 	}
 	if req.Body != nil {
-		iox.ReadAllContext(req.Context(), req.Body)
+		netxlite.ReadAllContext(req.Context(), req.Body)
 		req.Body.Close()
 	}
 	if txp.Err != nil {

internal/engine/experiment/hhfm/hhfm.go

@@ -20,8 +20,7 @@ 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/dialer"
-	"github.com/ooni/probe-cli/v3/internal/netxlite/errorsx"
-	"github.com/ooni/probe-cli/v3/internal/netxlite/iox"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 	"github.com/ooni/probe-cli/v3/internal/randx"
 )
 
@@ -165,7 +164,7 @@ func (m Measurer) Run(
 	// parse response body
 	var jsonHeaders JSONHeaders
 	if err := json.Unmarshal(data, &jsonHeaders); err != nil {
-		failure := errorsx.FailureJSONParseError
+		failure := netxlite.FailureJSONParseError
 		tk.Failure = &failure
 		tk.Tampering.Total = true
 		return nil // measurement did not fail, we measured tampering
@@ -182,7 +181,7 @@ func Transact(txp Transport, req *http.Request,
 	// make sure that we return a wrapped error here
 	resp, data, err := transact(txp, req, callbacks)
 	err = errorsxlegacy.SafeErrWrapperBuilder{
-		Error: err, Operation: errorsx.TopLevelOperation}.MaybeBuild()
+		Error: err, Operation: netxlite.TopLevelOperation}.MaybeBuild()
 	return resp, data, err
 }
 
@@ -199,7 +198,7 @@ func transact(txp Transport, req *http.Request,
 		return nil, nil, urlgetter.ErrHTTPRequestFailed
 	}
 	callbacks.OnProgress(0.75, "reading response body...")
-	data, err := iox.ReadAllContext(req.Context(), resp.Body)
+	data, err := netxlite.ReadAllContext(req.Context(), resp.Body)
 	callbacks.OnProgress(1.00, fmt.Sprintf("got reseponse body... %+v", err))
 	if err != nil {
 		return nil, nil, err

internal/engine/experiment/hhfm/hhfm_test.go

@@ -18,7 +18,7 @@ import (
 	"github.com/ooni/probe-cli/v3/internal/engine/mockable"
 	"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/netxlite/errorsx"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
 func TestNewExperimentMeasurer(t *testing.T) {
@@ -161,14 +161,14 @@ func TestCancelledContext(t *testing.T) {
 	if tk.Agent != "agent" {
 		t.Fatal("invalid Agent")
 	}
-	if *tk.Failure != errorsx.FailureInterrupted {
+	if *tk.Failure != netxlite.FailureInterrupted {
 		t.Fatal("invalid Failure")
 	}
 	if len(tk.Requests) != 1 {
 		t.Fatal("invalid Requests")
 	}
 	request := tk.Requests[0]
-	if *request.Failure != errorsx.FailureInterrupted {
+	if *request.Failure != netxlite.FailureInterrupted {
 		t.Fatal("invalid Requests[0].Failure")
 	}
 	if request.Request.Body.Value != "" {
@@ -480,7 +480,7 @@ func TestInvalidJSONBody(t *testing.T) {
 	if tk.Agent != "agent" {
 		t.Fatal("invalid Agent")
 	}
-	if *tk.Failure != errorsx.FailureJSONParseError {
+	if *tk.Failure != netxlite.FailureJSONParseError {
 		t.Fatal("invalid Failure")
 	}
 	if len(tk.Requests) != 1 {