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/netxlite/dnsx/dnsoverudp.go

@@ -1,69 +0,0 @@
-package dnsx
-
-import (
-	"context"
-	"net"
-	"time"
-)
-
-// Dialer is the network dialer interface assumed by this package.
-type Dialer interface {
-	DialContext(ctx context.Context, network, address string) (net.Conn, error)
-}
-
-// DNSOverUDP is a DNS over UDP RoundTripper.
-type DNSOverUDP struct {
-	dialer  Dialer
-	address string
-}
-
-// NewDNSOverUDP creates a DNSOverUDP instance.
-func NewDNSOverUDP(dialer Dialer, address string) *DNSOverUDP {
-	return &DNSOverUDP{dialer: dialer, address: address}
-}
-
-// RoundTrip implements RoundTripper.RoundTrip.
-func (t *DNSOverUDP) RoundTrip(ctx context.Context, query []byte) ([]byte, error) {
-	conn, err := t.dialer.DialContext(ctx, "udp", t.address)
-	if err != nil {
-		return nil, err
-	}
-	defer conn.Close()
-	// Use five seconds timeout like Bionic does. See
-	// https://labs.ripe.net/Members/baptiste_jonglez_1/persistent-dns-connections-for-reliability-and-performance
-	if err = conn.SetDeadline(time.Now().Add(5 * time.Second)); err != nil {
-		return nil, err
-	}
-	if _, err = conn.Write(query); err != nil {
-		return nil, err
-	}
-	reply := make([]byte, 1<<17)
-	var n int
-	n, err = conn.Read(reply)
-	if err != nil {
-		return nil, err
-	}
-	return reply[:n], nil
-}
-
-// RequiresPadding returns false for UDP according to RFC8467
-func (t *DNSOverUDP) RequiresPadding() bool {
-	return false
-}
-
-// Network returns the transport network (e.g., doh, dot)
-func (t *DNSOverUDP) Network() string {
-	return "udp"
-}
-
-// Address returns the upstream server address.
-func (t *DNSOverUDP) Address() string {
-	return t.address
-}
-
-// CloseIdleConnections closes idle connections.
-func (t *DNSOverUDP) CloseIdleConnections() {
-	// nothing to do
-}
-
-var _ DNSTransport = &DNSOverUDP{}