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 = `
 ##