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/dnsovertcp.go

@@ -0,0 +1,102 @@
+package netxlite
+
+import (
+	"context"
+	"errors"
+	"io"
+	"math"
+	"net"
+	"time"
+)
+
+// DialContextFunc is a generic function for dialing a connection.
+type DialContextFunc func(context.Context, string, string) (net.Conn, error)
+
+// DNSOverTCP is a DNS over TCP/TLS RoundTripper. Use NewDNSOverTCP
+// and NewDNSOverTLS to create specific instances that use plaintext
+// queries or encrypted queries over TLS.
+//
+// As a known bug, this implementation always creates a new connection
+// for each incoming query, thus increasing the response delay.
+type DNSOverTCP struct {
+	dial            DialContextFunc
+	address         string
+	network         string
+	requiresPadding bool
+}
+
+// NewDNSOverTCP creates a new DNSOverTCP transport.
+func NewDNSOverTCP(dial DialContextFunc, address string) *DNSOverTCP {
+	return &DNSOverTCP{
+		dial:            dial,
+		address:         address,
+		network:         "tcp",
+		requiresPadding: false,
+	}
+}
+
+// NewDNSOverTLS creates a new DNSOverTLS transport.
+func NewDNSOverTLS(dial DialContextFunc, address string) *DNSOverTCP {
+	return &DNSOverTCP{
+		dial:            dial,
+		address:         address,
+		network:         "dot",
+		requiresPadding: true,
+	}
+}
+
+// RoundTrip implements RoundTripper.RoundTrip.
+func (t *DNSOverTCP) RoundTrip(ctx context.Context, query []byte) ([]byte, error) {
+	if len(query) > math.MaxUint16 {
+		return nil, errors.New("query too long")
+	}
+	conn, err := t.dial(ctx, "tcp", t.address)
+	if err != nil {
+		return nil, err
+	}
+	defer conn.Close()
+	if err = conn.SetDeadline(time.Now().Add(10 * time.Second)); err != nil {
+		return nil, err
+	}
+	// Write request
+	buf := []byte{byte(len(query) >> 8)}
+	buf = append(buf, byte(len(query)))
+	buf = append(buf, query...)
+	if _, err = conn.Write(buf); err != nil {
+		return nil, err
+	}
+	// Read response
+	header := make([]byte, 2)
+	if _, err = io.ReadFull(conn, header); err != nil {
+		return nil, err
+	}
+	length := int(header[0])<<8 | int(header[1])
+	reply := make([]byte, length)
+	if _, err = io.ReadFull(conn, reply); err != nil {
+		return nil, err
+	}
+	return reply, nil
+}
+
+// RequiresPadding returns true for DoT and false for TCP
+// according to RFC8467.
+func (t *DNSOverTCP) RequiresPadding() bool {
+	return t.requiresPadding
+}
+
+// Network returns the transport network (e.g., doh, dot)
+func (t *DNSOverTCP) Network() string {
+	return t.network
+}
+
+// Address returns the upstream server address.
+func (t *DNSOverTCP) Address() string {
+	return t.address
+}
+
+// CloseIdleConnections closes idle connections.
+func (t *DNSOverTCP) CloseIdleConnections() {
+	// nothing to do
+}
+
+var _ DNSTransport = &DNSOverTCP{}