refactor(netxlite): hide details without breaking the rest of the tree (#454)

## Description

This PR continues the refactoring of `netx` under the following principles:

1. do not break the rest of the tree and do not engage in extensive tree-wide refactoring yet
2. move under `netxlite` clearly related subpackages (e.g., `iox`, `netxmocks`)
3. move into `internal/netxlite/internal` stuff that is clearly private of `netxlite`
4. hide implementation details in `netxlite` pending new factories
5. refactor `tls` code in `netxlite` to clearly separate `crypto/tls` code from `utls` code

After each commit, I run `go test -short -race ./...` locally. Each individual commit explains what it does. I will squash, but this operation will preserve the original commit titles, so this will give further insight on each step.

## Commits

* refactor: rename netxmocks -> netxlite/mocks

Part of https://github.com/ooni/probe/issues/1591

* refactor: rename quicx -> netxlite/quicx

See https://github.com/ooni/probe/issues/1591

* refactor: rename iox -> netxlite/iox

Regenerate sources and make sure the tests pass.

See https://github.com/ooni/probe/issues/1591.

* refactor(iox): move MockableReader to netxlite/mocks

See https://github.com/ooni/probe/issues/1591

* refactor(netxlite): generator is an implementation detail

See https://github.com/ooni/probe/issues/1591

* refactor(netxlite): separate tls and utls code

See https://github.com/ooni/probe/issues/1591

* refactor(netxlite): hide most types but keep old names as legacy

With this change we avoid breaking the rest of the tree, but we start
hiding some implementation details a bit. Factories will follow.

See https://github.com/ooni/probe/issues/1591

internal/netxlite/mocks/conn_test.go

@@ -0,0 +1,126 @@
+package mocks
+
+import (
+	"errors"
+	"net"
+	"testing"
+	"time"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestConnReadWorks(t *testing.T) {
+	expected := errors.New("mocked error")
+	c := &Conn{
+		MockRead: func(b []byte) (int, error) {
+			return 0, expected
+		},
+	}
+	count, err := c.Read(make([]byte, 128))
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+	if count != 0 {
+		t.Fatal("expected 0 bytes")
+	}
+}
+
+func TestConnWriteWorks(t *testing.T) {
+	expected := errors.New("mocked error")
+	c := &Conn{
+		MockWrite: func(b []byte) (int, error) {
+			return 0, expected
+		},
+	}
+	count, err := c.Write(make([]byte, 128))
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+	if count != 0 {
+		t.Fatal("expected 0 bytes")
+	}
+}
+
+func TestConnCloseWorks(t *testing.T) {
+	expected := errors.New("mocked error")
+	c := &Conn{
+		MockClose: func() error {
+			return expected
+		},
+	}
+	err := c.Close()
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+}
+
+func TestConnLocalAddrWorks(t *testing.T) {
+	expected := &net.TCPAddr{
+		IP:   net.IPv6loopback,
+		Port: 1234,
+	}
+	c := &Conn{
+		MockLocalAddr: func() net.Addr {
+			return expected
+		},
+	}
+	out := c.LocalAddr()
+	if diff := cmp.Diff(expected, out); diff != "" {
+		t.Fatal(diff)
+	}
+}
+
+func TestConnRemoteAddrWorks(t *testing.T) {
+	expected := &net.TCPAddr{
+		IP:   net.IPv6loopback,
+		Port: 1234,
+	}
+	c := &Conn{
+		MockRemoteAddr: func() net.Addr {
+			return expected
+		},
+	}
+	out := c.RemoteAddr()
+	if diff := cmp.Diff(expected, out); diff != "" {
+		t.Fatal(diff)
+	}
+}
+
+func TestConnSetDeadline(t *testing.T) {
+	expected := errors.New("mocked error")
+	c := &Conn{
+		MockSetDeadline: func(t time.Time) error {
+			return expected
+		},
+	}
+	err := c.SetDeadline(time.Time{})
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected", err)
+	}
+}
+
+func TestConnSetReadDeadline(t *testing.T) {
+	expected := errors.New("mocked error")
+	c := &Conn{
+		MockSetReadDeadline: func(t time.Time) error {
+			return expected
+		},
+	}
+	err := c.SetReadDeadline(time.Time{})
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected", err)
+	}
+}
+
+func TestConnSetWriteDeadline(t *testing.T) {
+	expected := errors.New("mocked error")
+	c := &Conn{
+		MockSetWriteDeadline: func(t time.Time) error {
+			return expected
+		},
+	}
+	err := c.SetWriteDeadline(time.Time{})
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected", err)
+	}
+}