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/experiment/webconnectivity/connects.go

@@ -0,0 +1,58 @@
+package webconnectivity
+
+import (
+	"context"
+	"net/url"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/experiment/urlgetter"
+	"github.com/ooni/probe-cli/v3/internal/engine/model"
+)
+
+// ConnectsConfig contains the config for Connects
+type ConnectsConfig struct {
+	Session       model.ExperimentSession
+	TargetURL     *url.URL
+	URLGetterURLs []string
+}
+
+// TODO(bassosimone): we should normalize the timings
+
+// ConnectsResult contains the results of Connects
+type ConnectsResult struct {
+	AllKeys   []urlgetter.TestKeys
+	Successes int
+	Total     int
+}
+
+// Connects performs 0..N connects (either using TCP or TLS) to
+// check whether the resolved endpoints are reachable.
+func Connects(ctx context.Context, config ConnectsConfig) (out ConnectsResult) {
+	out.AllKeys = []urlgetter.TestKeys{}
+	multi := urlgetter.Multi{Session: config.Session}
+	inputs := []urlgetter.MultiInput{}
+	for _, url := range config.URLGetterURLs {
+		inputs = append(inputs, urlgetter.MultiInput{
+			Config: urlgetter.Config{
+				TLSServerName: config.TargetURL.Hostname(),
+			},
+			Target: url,
+		})
+	}
+	outputs := multi.Collect(ctx, inputs, "check", ConnectsNoCallbacks{})
+	for multiout := range outputs {
+		out.AllKeys = append(out.AllKeys, multiout.TestKeys)
+		for _, entry := range multiout.TestKeys.TCPConnect {
+			if entry.Status.Success {
+				out.Successes++
+			}
+			out.Total++
+		}
+	}
+	return
+}
+
+// ConnectsNoCallbacks suppresses the callbacks
+type ConnectsNoCallbacks struct{}
+
+// OnProgress implements ExperimentCallbacks.OnProgress
+func (ConnectsNoCallbacks) OnProgress(percentage float64, message string) {}