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/dnsencoder_test.go

@@ -0,0 +1,103 @@
+package netxlite
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+func TestDNSEncoder(t *testing.T) {
+
+	t.Run("encode A", func(t *testing.T) {
+		e := &DNSEncoderMiekg{}
+		data, err := e.Encode("x.org", dns.TypeA, false)
+		if err != nil {
+			t.Fatal(err)
+		}
+		dnsValidateEncodedQueryBytes(t, data, byte(dns.TypeA))
+	})
+
+	t.Run("encode AAAA", func(t *testing.T) {
+		e := &DNSEncoderMiekg{}
+		data, err := e.Encode("x.org", dns.TypeAAAA, false)
+		if err != nil {
+			t.Fatal(err)
+		}
+		dnsValidateEncodedQueryBytes(t, data, byte(dns.TypeA))
+	})
+
+	t.Run("encode padding", func(t *testing.T) {
+		// The purpose of this unit test is to make sure that for a wide
+		// array of values we obtain the right query size.
+		getquerylen := func(domainlen int, padding bool) int {
+			e := &DNSEncoderMiekg{}
+			data, err := e.Encode(
+				// This is not a valid name because it ends up being way
+				// longer than 255 octets. However, the library is allowing
+				// us to generate such name and we are not going to send
+				// it on the wire. Also, we check below that the query that
+				// we generate is long enough, so we should be good.
+				dns.Fqdn(strings.Repeat("x.", domainlen)),
+				dns.TypeA, padding,
+			)
+			if err != nil {
+				t.Fatal(err)
+			}
+			return len(data)
+		}
+		for domainlen := 1; domainlen <= 4000; domainlen++ {
+			vanillalen := getquerylen(domainlen, false)
+			paddedlen := getquerylen(domainlen, true)
+			if vanillalen < domainlen {
+				t.Fatal("vanillalen is smaller than domainlen")
+			}
+			if (paddedlen % dnsPaddingDesiredBlockSize) != 0 {
+				t.Fatal("paddedlen is not a multiple of PaddingDesiredBlockSize")
+			}
+			if paddedlen < vanillalen {
+				t.Fatal("paddedlen is smaller than vanillalen")
+			}
+		}
+	})
+}
+
+// dnsValidateEncodedQueryBytes validates the query serialized in data
+// for the given query type qtype (e.g., dns.TypeAAAA).
+func dnsValidateEncodedQueryBytes(t *testing.T, data []byte, qtype byte) {
+	// skipping over the query ID
+	if data[2] != 1 {
+		t.Fatal("FLAGS should only have RD set")
+	}
+	if data[3] != 0 {
+		t.Fatal("RA|Z|Rcode should be zero")
+	}
+	if data[4] != 0 || data[5] != 1 {
+		t.Fatal("QCOUNT high should be one")
+	}
+	if data[6] != 0 || data[7] != 0 {
+		t.Fatal("ANCOUNT should be zero")
+	}
+	if data[8] != 0 || data[9] != 0 {
+		t.Fatal("NSCOUNT should be zero")
+	}
+	if data[10] != 0 || data[11] != 0 {
+		t.Fatal("ARCOUNT should be zero")
+	}
+	t.Log(data[12])
+	if data[12] != 1 || data[13] != byte('x') {
+		t.Fatal("The name does not contain 1:x")
+	}
+	if data[14] != 3 || data[15] != byte('o') || data[16] != byte('r') || data[17] != byte('g') {
+		t.Fatal("The name does not contain 3:org")
+	}
+	if data[18] != 0 {
+		t.Fatal("The name does not terminate where expected")
+	}
+	if data[19] != 0 && data[20] != qtype {
+		t.Fatal("The query is not for the expected type")
+	}
+	if data[21] != 0 && data[22] != 1 {
+		t.Fatal("The query is not IN")
+	}
+}