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/mocks/dnsdecoder_test.go

@@ -0,0 +1,42 @@
+package mocks
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+func TestDNSDecoder(t *testing.T) {
+	t.Run("DecodeLookupHost", func(t *testing.T) {
+		expected := errors.New("mocked error")
+		e := &DNSDecoder{
+			MockDecodeLookupHost: func(qtype uint16, reply []byte) ([]string, error) {
+				return nil, expected
+			},
+		}
+		out, err := e.DecodeLookupHost(dns.TypeA, make([]byte, 17))
+		if !errors.Is(err, expected) {
+			t.Fatal("unexpected err", err)
+		}
+		if out != nil {
+			t.Fatal("unexpected out")
+		}
+	})
+
+	t.Run("DecodeHTTPS", func(t *testing.T) {
+		expected := errors.New("mocked error")
+		e := &DNSDecoder{
+			MockDecodeHTTPS: func(reply []byte) (*HTTPSSvc, error) {
+				return nil, expected
+			},
+		}
+		out, err := e.DecodeHTTPS(make([]byte, 17))
+		if !errors.Is(err, expected) {
+			t.Fatal("unexpected err", err)
+		}
+		if out != nil {
+			t.Fatal("unexpected out")
+		}
+	})
+}