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/certifi.go

@@ -1,10 +1,10 @@
 // Code generated by go generate; DO NOT EDIT.
-// 2021-06-25 12:32:42.759674 +0200 CEST m=+0.458880709
+// 2021-07-02 14:26:19.300018 +0200 CEST m=+0.978891751
 // https://curl.haxx.se/ca/cacert.pem
 
 package netxlite
 
-//go:generate go run certifigen.go "https://curl.haxx.se/ca/cacert.pem"
+//go:generate go run ./generator/ "https://curl.haxx.se/ca/cacert.pem"
 
 const pemcerts string = `
 ##

internal/netxlite/generator/main.go

@@ -1,5 +1,3 @@
-// +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/.
@@ -30,7 +28,7 @@ var tmpl = template.Must(template.New("").Parse(`// Code generated by go generat
 
 package netxlite
 
-//go:generate go run certifigen.go "{{ .URL }}"
+//{{ .GoGenerate }} go run ./generator/ "{{ .URL }}"
 
 const pemcerts string = ` + "`" + `
 {{ .Bundle }}
@@ -39,7 +37,7 @@ const pemcerts string = ` + "`" + `
 
 func main() {
 	if len(os.Args) != 2 || !strings.HasPrefix(os.Args[1], "https://") {
-		log.Fatal("usage: go run generate.go <url>")
+		log.Fatal("usage: go run ./generator/ <url>")
 	}
 	url := os.Args[1]
 
@@ -68,13 +66,15 @@ func main() {
 	}
 
 	err = tmpl.Execute(fp, struct {
-		Timestamp time.Time
-		URL       string
-		Bundle    string
+		Bundle     string
+		GoGenerate string
+		Timestamp  time.Time
+		URL        string
 	}{
-		Timestamp: time.Now(),
-		URL:       url,
-		Bundle:    string(bundle),
+		Bundle:     string(bundle),
+		GoGenerate: "go:generate",
+		Timestamp:  time.Now(),
+		URL:        url,
 	})
 	if err != nil {
 		log.Fatal(err)