chore: merge probe-engine into probe-cli (#201)

This is how I did it:

1. `git clone https://github.com/ooni/probe-engine internal/engine`

2. ```
(cd internal/engine && git describe --tags)
v0.23.0
```

3. `nvim go.mod` (merging `go.mod` with `internal/engine/go.mod`

4. `rm -rf internal/.git internal/engine/go.{mod,sum}`

5. `git add internal/engine`

6. `find . -type f -name \*.go -exec sed -i 's@/ooni/probe-engine@/ooni/probe-cli/v3/internal/engine@g' {} \;`

7. `go build ./...` (passes)

8. `go test -race ./...` (temporary failure on RiseupVPN)

9. `go mod tidy`

10. this commit message

Once this piece of work is done, we can build a new version of `ooniprobe` that
is using `internal/engine` directly. We need to do more work to ensure all the
other functionality in `probe-engine` (e.g. making mobile packages) are still WAI.

Part of https://github.com/ooni/probe/issues/1335

internal/engine/netx/dialer/proxy.go

@@ -0,0 +1,94 @@
+package dialer
+
+import (
+	"context"
+	"errors"
+	"net"
+	"net/url"
+
+	"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.
+//
+// As a special case, you can force a proxy to be used only extemporarily. To this end,
+// you can use the WithProxyURL function, to store the proxy URL in the context. This
+// will take precedence over any otherwise configured proxy. The use case for this
+// functionality is when you need a tunnel to contact OONI probe services.
+type ProxyDialer struct {
+	Dialer
+	ProxyURL *url.URL
+}
+
+type proxyKey struct{}
+
+// ContextProxyURL retrieves the proxy URL from the context. This is mainly used
+// to force a tunnel when we fail contacting OONI probe services otherwise.
+func ContextProxyURL(ctx context.Context) *url.URL {
+	url, _ := ctx.Value(proxyKey{}).(*url.URL)
+	return url
+}
+
+// WithProxyURL assigns the proxy URL to the context
+func WithProxyURL(ctx context.Context, url *url.URL) context.Context {
+	return context.WithValue(ctx, proxyKey{}, url)
+}
+
+// DialContext implements Dialer.DialContext
+func (d ProxyDialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
+	url := ContextProxyURL(ctx) // context URL takes precendence
+	if url == nil {
+		url = d.ProxyURL
+	}
+	if url == nil {
+		return d.Dialer.DialContext(ctx, network, address)
+	}
+	if url.Scheme != "socks5" {
+		return nil, errors.New("Scheme is not socks5")
+	}
+	// the code at proxy/socks5.go never fails; see https://git.io/JfJ4g
+	child, _ := proxy.SOCKS5(
+		network, url.Host, nil, proxyDialerWrapper{Dialer: 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) {
+	connch := make(chan net.Conn)
+	errch := make(chan error, 1)
+	go func() {
+		conn, err := child.Dial(network, address)
+		if err != nil {
+			errch <- err
+			return
+		}
+		select {
+		case connch <- conn:
+		default:
+			conn.Close()
+		}
+	}()
+	select {
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	case err := <-errch:
+		return nil, err
+	case conn := <-connch:
+		return conn, nil
+	}
+}
+
+// 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 {
+	Dialer
+}
+
+func (d proxyDialerWrapper) Dial(network, address string) (net.Conn, error) {
+	return d.DialContext(context.Background(), network, address)
+}