refactor(netxlite): move generator in a subpackage

This way, we don't need to use `// +build ignore`, which prevents
the editor from linting the file itself.

Work part of https://github.com/ooni/probe/issues/1505.

internal/netxlite/generator/main.go

@@ -0,0 +1,86 @@
+// 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 (
+	"context"
+	"crypto/x509"
+	"log"
+	"net/http"
+	"os"
+	"strings"
+	"text/template"
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/iox"
+)
+
+var tmpl = template.Must(template.New("").Parse(`// Code generated by go generate; DO NOT EDIT.
+// {{ .Timestamp }}
+// {{ .URL }}
+
+package netxlite
+
+//{{ .GoGenerate }} go run ./generator/ "{{ .URL }}"
+
+const pemcerts string = ` + "`" + `
+{{ .Bundle }}
+` + "`" + `
+`))
+
+func main() {
+	if len(os.Args) != 2 || !strings.HasPrefix(os.Args[1], "https://") {
+		log.Fatal("usage: go run ./generator/ <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 := iox.ReadAllContext(context.Background(), 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 {
+		Bundle     string
+		GoGenerate string
+		Timestamp  time.Time
+		URL        string
+	}{
+		Bundle:     string(bundle),
+		GoGenerate: "go:generate",
+		Timestamp:  time.Now(),
+		URL:        url,
+	})
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	if err := fp.Close(); err != nil {
+		log.Fatal(err)
+	}
+}