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/netx/gocertifi/generate.go

@@ -0,0 +1,95 @@
+// +build ignore
+
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at https://mozilla.org/MPL/2.0/.
+//
+// Forked from github.com/certifi/gocertifi <https://git.io/JJjmG>.
+//
+// This script should not be invoked directly, rather it should be
+// executed by running go generate ./... from toplevel dir.
+
+package main
+
+import (
+	"crypto/x509"
+	"io/ioutil"
+	"log"
+	"net/http"
+	"os"
+	"strings"
+	"text/template"
+	"time"
+)
+
+var tmpl = template.Must(template.New("").Parse(`// Code generated by go generate; DO NOT EDIT.
+// {{ .Timestamp }}
+// {{ .URL }}
+
+package gocertifi
+
+//go:generate go run generate.go "{{ .URL }}"
+
+import "crypto/x509"
+
+const pemcerts string = ` + "`" + `
+{{ .Bundle }}
+` + "`" + `
+
+// CACerts builds an X.509 certificate pool containing the
+// certificate bundle from {{ .URL }} fetch on {{ .Timestamp }}.
+// Returns nil on error along with an appropriate error code.
+func CACerts() (*x509.CertPool, error) {
+	pool := x509.NewCertPool()
+	pool.AppendCertsFromPEM([]byte(pemcerts))
+	return pool, nil
+}
+`))
+
+func main() {
+	if len(os.Args) != 2 || !strings.HasPrefix(os.Args[1], "https://") {
+		log.Fatal("usage: go run generate.go <url>")
+	}
+	url := os.Args[1]
+
+	resp, err := http.Get(url)
+	if err != nil {
+		log.Fatal(err)
+	}
+	if resp.StatusCode != 200 {
+		log.Fatal("expected 200, got", resp.StatusCode)
+	}
+	defer resp.Body.Close()
+
+	bundle, err := ioutil.ReadAll(resp.Body)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	pool := x509.NewCertPool()
+	if !pool.AppendCertsFromPEM(bundle) {
+		log.Fatalf("can't parse certificates from %s", url)
+	}
+
+	fp, err := os.Create("certifi.go")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	err = tmpl.Execute(fp, struct {
+		Timestamp time.Time
+		URL       string
+		Bundle    string
+	}{
+		Timestamp: time.Now(),
+		URL:       url,
+		Bundle:    string(bundle),
+	})
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	if err := fp.Close(); err != nil {
+		log.Fatal(err)
+	}
+}