refactor(errorsx): improve errno generating code (#473)

No functional change, as it's clearly obvious from the output.

While there, also rename the generator for certifi. We are planning
on merging errorsx into netxlite. The first step is to give different
names to the code generating programs.

See https://github.com/ooni/probe/issues/1591

internal/netxlite/certifi.go

@@ -1,10 +1,10 @@
 // Code generated by go generate; DO NOT EDIT.
-// 2021-09-05 14:01:12.627844 +0200 CEST m=+0.441951210
+// 2021-09-07 12:37:42.667616 +0200 CEST m=+1.355122543
 // https://curl.haxx.se/ca/cacert.pem
 
 package netxlite
 
-//go:generate go run ./internal/generator/ "https://curl.haxx.se/ca/cacert.pem"
+//go:generate go run ./internal/gencertifi/ "https://curl.haxx.se/ca/cacert.pem"
 
 const pemcerts string = `
 ##

internal/netxlite/internal/gencertifi/main.go

@@ -28,7 +28,7 @@ var tmpl = template.Must(template.New("").Parse(`// Code generated by go generat
 
 package netxlite
 
-//{{ .GoGenerate }} go run ./internal/generator/ "{{ .URL }}"
+//{{ .GoGenerate }} go run ./internal/gencertifi/ "{{ .URL }}"
 
 const pemcerts string = ` + "`" + `
 {{ .Bundle }}