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/dash/download.go

@@ -0,0 +1,74 @@
+package dash
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"time"
+)
+
+type downloadDeps interface {
+	HTTPClient() *http.Client
+	NewHTTPRequest(method string, url string, body io.Reader) (*http.Request, error)
+	ReadAll(r io.Reader) ([]byte, error)
+	Scheme() string
+	UserAgent() string
+}
+
+type downloadConfig struct {
+	authorization string
+	begin         time.Time
+	currentRate   int64
+	deps          downloadDeps
+	elapsedTarget int64
+	fqdn          string
+}
+
+type downloadResult struct {
+	elapsed      float64
+	received     int64
+	requestTicks float64
+	serverURL    string
+	timestamp    int64
+}
+
+func download(ctx context.Context, config downloadConfig) (downloadResult, error) {
+	nbytes := (config.currentRate * 1000 * config.elapsedTarget) >> 3
+	var URL url.URL
+	URL.Scheme = config.deps.Scheme()
+	URL.Host = config.fqdn
+	URL.Path = fmt.Sprintf("%s%d", downloadPath, nbytes)
+	req, err := config.deps.NewHTTPRequest("GET", URL.String(), nil)
+	var result downloadResult
+	if err != nil {
+		return result, err
+	}
+	result.serverURL = URL.String()
+	req.Header.Set("User-Agent", config.deps.UserAgent())
+	req.Header.Set("Authorization", config.authorization)
+	savedTicks := time.Now()
+	resp, err := config.deps.HTTPClient().Do(req.WithContext(ctx))
+	if err != nil {
+		return result, err
+	}
+	if resp.StatusCode != 200 {
+		return result, errHTTPRequestFailed
+	}
+	defer resp.Body.Close()
+	data, err := config.deps.ReadAll(resp.Body)
+	if err != nil {
+		return result, err
+	}
+	// Implementation note: MK contains a comment that says that Neubot uses
+	// the elapsed time since when we start receiving the response but it
+	// turns out that Neubot and MK do the same. So, we do what they do. At
+	// the same time, we are currently not able to include the overhead that
+	// is caused by HTTP headers etc. So, we're a bit less precise.
+	result.elapsed = time.Now().Sub(savedTicks).Seconds()
+	result.received = int64(len(data))
+	result.requestTicks = savedTicks.Sub(config.begin).Seconds()
+	result.timestamp = time.Now().Unix()
+	return result, nil
+}