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/mocks/dnsencoder_test.go

@@ -1,26 +0,0 @@
-package mocks
-
-import (
-	"errors"
-	"testing"
-
-	"github.com/miekg/dns"
-)
-
-func TestDNSEncoder(t *testing.T) {
-	t.Run("Encode", func(t *testing.T) {
-		expected := errors.New("mocked error")
-		e := &DNSEncoder{
-			MockEncode: func(domain string, qtype uint16, padding bool) ([]byte, error) {
-				return nil, expected
-			},
-		}
-		out, err := e.Encode("dns.google", dns.TypeA, true)
-		if !errors.Is(err, expected) {
-			t.Fatal("unexpected err", err)
-		}
-		if out != nil {
-			t.Fatal("unexpected out")
-		}
-	})
-}