refactor(session.go): replace engine/netx with netxlite (#767)

This diff replaces engine/netx code with netxlite code in
the engine/session.go file. To this end, we needed to move
some code from engine/netx to netxlite. While there, we
did review and improve the unit tests.

A notable change in this diff is (or seems to be) that in
engine/session.go we're not filtering for bogons anymore so
that, in principle, we could believe a resolver returning
to us bogon IP addresses for OONI services. However, I did
not bother with changing bogons filtering because the
sessionresolver package is already filtering for bogons,
so it is actually okay to avoid doing that again the
session.go code. See:

https://github.com/ooni/probe-cli/blob/v3.15.0-alpha.1/internal/engine/internal/sessionresolver/resolvermaker.go#L88

There are two reference issues for this cleanup:

1. https://github.com/ooni/probe/issues/2115

2. https://github.com/ooni/probe/issues/2121

internal/engine/netx/dialer/proxy.go

@@ -1,56 +1,5 @@
 package dialer
 
-import (
-	"context"
-	"errors"
-	"net"
-	"net/url"
+import "github.com/ooni/probe-cli/v3/internal/netxlite"
 
-	"github.com/ooni/probe-cli/v3/internal/model"
-	"golang.org/x/net/proxy"
-)
-
-// proxyDialer is a dialer that uses a proxy. If the ProxyURL is not configured, this
-// dialer is a passthrough for the next Dialer in chain. Otherwise, it will internally
-// create a SOCKS5 dialer that will connect to the proxy using the underlying Dialer.
-type proxyDialer struct {
-	model.Dialer
-	ProxyURL *url.URL
-}
-
-// ErrProxyUnsupportedScheme indicates we don't support a protocol scheme.
-var ErrProxyUnsupportedScheme = errors.New("proxy: unsupported scheme")
-
-// DialContext implements Dialer.DialContext
-func (d *proxyDialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
-	url := d.ProxyURL
-	if url == nil {
-		return d.Dialer.DialContext(ctx, network, address)
-	}
-	if url.Scheme != "socks5" {
-		return nil, ErrProxyUnsupportedScheme
-	}
-	// the code at proxy/socks5.go never fails; see https://git.io/JfJ4g
-	child, _ := proxy.SOCKS5(
-		network, url.Host, nil, &proxyDialerWrapper{d.Dialer})
-	return d.dial(ctx, child, network, address)
-}
-
-func (d *proxyDialer) dial(
-	ctx context.Context, child proxy.Dialer, network, address string) (net.Conn, error) {
-	cd := child.(proxy.ContextDialer) // will work
-	return cd.DialContext(ctx, network, address)
-}
-
-// proxyDialerWrapper is required because SOCKS5 expects a Dialer.Dial type but internally
-// it checks whether DialContext is available and prefers that. So, we need to use this
-// structure to cast our inner Dialer the way in which SOCKS5 likes it.
-//
-// See https://git.io/JfJ4g.
-type proxyDialerWrapper struct {
-	model.Dialer
-}
-
-func (d *proxyDialerWrapper) Dial(network, address string) (net.Conn, error) {
-	panic(errors.New("proxyDialerWrapper.Dial should not be called directly"))
-}
+type proxyDialer = netxlite.MaybeProxyDialer