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/cmd/oohelperd/internal/http.go

@@ -0,0 +1,67 @@
+package internal
+
+import (
+	"context"
+	"io"
+	"io/ioutil"
+	"net/http"
+	"strings"
+	"sync"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/experiment/webconnectivity"
+)
+
+// CtrlHTTPResponse is the result of the HTTP check performed by
+// the Web Connectivity test helper.
+type CtrlHTTPResponse = webconnectivity.ControlHTTPRequestResult
+
+// HTTPConfig configures the HTTP check.
+type HTTPConfig struct {
+	Client            *http.Client
+	Headers           map[string][]string
+	MaxAcceptableBody int64
+	Out               chan CtrlHTTPResponse
+	URL               string
+	Wg                *sync.WaitGroup
+}
+
+// HTTPDo performs the HTTP check.
+func HTTPDo(ctx context.Context, config *HTTPConfig) {
+	defer config.Wg.Done()
+	req, err := http.NewRequestWithContext(ctx, "GET", config.URL, nil)
+	if err != nil {
+		config.Out <- CtrlHTTPResponse{Failure: newfailure(err)}
+		return
+	}
+	// The original test helper failed with extra headers while here
+	// we're implementing (for now?) a more liberal approach.
+	for k, vs := range config.Headers {
+		switch strings.ToLower(k) {
+		case "user-agent":
+		case "accept":
+		case "accept-language":
+			for _, v := range vs {
+				req.Header.Add(k, v)
+			}
+		}
+	}
+	resp, err := config.Client.Do(req)
+	if err != nil {
+		config.Out <- CtrlHTTPResponse{Failure: newfailure(err)}
+		return
+	}
+	defer resp.Body.Close()
+	headers := make(map[string]string)
+	for k := range resp.Header {
+		headers[k] = resp.Header.Get(k)
+	}
+	reader := &io.LimitedReader{R: resp.Body, N: config.MaxAcceptableBody}
+	data, err := ioutil.ReadAll(reader)
+	config.Out <- CtrlHTTPResponse{
+		BodyLength: int64(len(data)),
+		Failure:    newfailure(err),
+		StatusCode: int64(resp.StatusCode),
+		Headers:    headers,
+		Title:      webconnectivity.GetTitle(string(data)),
+	}
+}