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/internal/mlablocate/mlablocate.go

@@ -0,0 +1,79 @@
+// Package mlablocate contains a locate.measurementlab.net client.
+package mlablocate
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"net/url"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/model"
+)
+
+// Client is a locate.measurementlab.net client.
+type Client struct {
+	HTTPClient *http.Client
+	Hostname   string
+	Logger     model.Logger
+	Scheme     string
+	UserAgent  string
+}
+
+// NewClient creates a new locate.measurementlab.net client.
+func NewClient(httpClient *http.Client, logger model.Logger, userAgent string) *Client {
+	return &Client{
+		HTTPClient: httpClient,
+		Hostname:   "locate.measurementlab.net",
+		Logger:     logger,
+		Scheme:     "https",
+		UserAgent:  userAgent,
+	}
+}
+
+// Result is a result of a query to locate.measurementlab.net.
+type Result struct {
+	City    string   `json:"city"`
+	Country string   `json:"country"`
+	IP      []string `json:"ip"`
+	FQDN    string   `json:"fqdn"`
+	Site    string   `json:"site"`
+}
+
+// Query performs a locate.measurementlab.net query.
+func (c *Client) Query(ctx context.Context, tool string) (Result, error) {
+	URL := &url.URL{
+		Scheme: c.Scheme,
+		Host:   c.Hostname,
+		Path:   tool,
+	}
+	req, err := http.NewRequestWithContext(ctx, "GET", URL.String(), nil)
+	if err != nil {
+		return Result{}, err
+	}
+	req.Header.Add("User-Agent", c.UserAgent)
+	c.Logger.Debugf("mlablocate: GET %s", URL.String())
+	resp, err := c.HTTPClient.Do(req)
+	if err != nil {
+		return Result{}, err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != 200 {
+		return Result{}, fmt.Errorf("mlablocate: non-200 status code: %d", resp.StatusCode)
+	}
+	data, err := ioutil.ReadAll(resp.Body)
+	if err != nil {
+		return Result{}, err
+	}
+	c.Logger.Debugf("mlablocate: %s", string(data))
+	var result Result
+	if err := json.Unmarshal(data, &result); err != nil {
+		return Result{}, err
+	}
+	if result.FQDN == "" {
+		return Result{}, errors.New("mlablocate: returned empty FQDN")
+	}
+	return result, nil
+}