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/quicx/quicx.go

@@ -0,0 +1,40 @@
+// Package quicx contains lucas-clemente/quic-go extensions.
+//
+// This code introduces the UDPLikeConn, whose documentation explain
+// why we need to introduce this new type. We could not put this
+// code inside an existing package because it's used (as of 20 Aug 2021)
+// by the netxlite package as well as by the mocks package.
+package quicx
+
+import (
+	"net"
+	"syscall"
+)
+
+// UDPLikeConn is a net.PacketConn with some extra functions
+// required to convince the QUIC library (lucas-clemente/quic-go)
+// to inflate the receive buffer of the connection.
+//
+// The QUIC library will treat this connection as a "dumb"
+// net.PacketConn, calling its ReadFrom and WriteTo methods
+// as opposed to more advanced methods that are available
+// under Linux and FreeBSD and improve the performance.
+//
+// It seems fine to avoid performance optimizations, because
+// they would complicate the implementation on our side and
+// our use cases (blocking and heavy throttling) do not seem
+// to require such optimizations.
+//
+// See https://github.com/ooni/probe/issues/1754 for a more
+// comprehensive discussion of UDPLikeConn.
+type UDPLikeConn interface {
+	// An UDPLikeConn is a net.PacketConn conn.
+	net.PacketConn
+
+	// SetReadBuffer allows setting the read buffer.
+	SetReadBuffer(bytes int) error
+
+	// SyscallConn returns a conn suitable for calling syscalls,
+	// which is also instrumental to setting the read buffer.
+	SyscallConn() (syscall.RawConn, error)
+}