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/utls_test.go

@@ -0,0 +1,28 @@
+package netxlite
+
+import (
+	"context"
+	"crypto/tls"
+	"net"
+	"testing"
+
+	utls "gitlab.com/yawning/utls.git"
+)
+
+func TestUTLSHandshakerChrome(t *testing.T) {
+	h := &tlsHandshakerConfigurable{
+		NewConn: NewConnUTLS(&utls.HelloChrome_Auto),
+	}
+	cfg := &tls.Config{ServerName: "google.com"}
+	conn, err := net.Dial("tcp", "google.com:443")
+	if err != nil {
+		t.Fatal("unexpected error", err)
+	}
+	conn, _, err = h.Handshake(context.Background(), conn, cfg)
+	if err != nil {
+		t.Fatal("unexpected error", err)
+	}
+	if conn == nil {
+		t.Fatal("nil connection")
+	}
+}