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/bytecounter/conn_test.go

@@ -4,12 +4,12 @@ import (
 	"errors"
 	"testing"
 
-	"github.com/ooni/probe-cli/v3/internal/netxmocks"
+	"github.com/ooni/probe-cli/v3/internal/netxlite/mocks"
 )
 
 func TestConnWorksOnSuccess(t *testing.T) {
 	counter := New()
-	underlying := &netxmocks.Conn{
+	underlying := &mocks.Conn{
 		MockRead: func(b []byte) (int, error) {
 			return 10, nil
 		},
@@ -39,7 +39,7 @@ func TestConnWorksOnFailure(t *testing.T) {
 	readError := errors.New("read error")
 	writeError := errors.New("write error")
 	counter := New()
-	underlying := &netxmocks.Conn{
+	underlying := &mocks.Conn{
 		MockRead: func(b []byte) (int, error) {
 			return 0, readError
 		},