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/collect.go

@@ -0,0 +1,57 @@
+package dash
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/url"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/model"
+)
+
+type collectDeps interface {
+	HTTPClient() *http.Client
+	JSONMarshal(v interface{}) ([]byte, error)
+	Logger() model.Logger
+	NewHTTPRequest(method string, url string, body io.Reader) (*http.Request, error)
+	ReadAll(r io.Reader) ([]byte, error)
+	Scheme() string
+	UserAgent() string
+}
+
+func collect(ctx context.Context, fqdn, authorization string,
+	results []clientResults, deps collectDeps) error {
+	data, err := deps.JSONMarshal(results)
+	if err != nil {
+		return err
+	}
+	deps.Logger().Debugf("dash: body: %s", string(data))
+	var URL url.URL
+	URL.Scheme = deps.Scheme()
+	URL.Host = fqdn
+	URL.Path = collectPath
+	req, err := deps.NewHTTPRequest("POST", URL.String(), bytes.NewReader(data))
+	if err != nil {
+		return err
+	}
+	req.Header.Set("User-Agent", deps.UserAgent())
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Authorization", authorization)
+	resp, err := deps.HTTPClient().Do(req.WithContext(ctx))
+	if err != nil {
+		return err
+	}
+	if resp.StatusCode != 200 {
+		return errHTTPRequestFailed
+	}
+	defer resp.Body.Close()
+	data, err = deps.ReadAll(resp.Body)
+	if err != nil {
+		return err
+	}
+	deps.Logger().Debugf("dash: body: %s", string(data))
+	var serverResults []serverResults
+	return json.Unmarshal(data, &serverResults)
+}