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/cmd/oohelperd/internal/webconnectivity/http.go

@@ -8,7 +8,7 @@ import (
 	"sync"
 
 	"github.com/ooni/probe-cli/v3/internal/engine/experiment/webconnectivity"
-	"github.com/ooni/probe-cli/v3/internal/netxlite/iox"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
 // CtrlHTTPResponse is the result of the HTTP check performed by
@@ -62,7 +62,7 @@ func HTTPDo(ctx context.Context, config *HTTPConfig) {
 		headers[k] = resp.Header.Get(k)
 	}
 	reader := &io.LimitedReader{R: resp.Body, N: config.MaxAcceptableBody}
-	data, err := iox.ReadAllContext(ctx, reader)
+	data, err := netxlite.ReadAllContext(ctx, reader)
 	config.Out <- CtrlHTTPResponse{
 		BodyLength: int64(len(data)),
 		Failure:    newfailure(err),