refactor: merge dnsx and errorsx into netxlite (#517)

When preparing a tutorial for netxlite, I figured it is easier
to tell people "hey, this is the package you should use for all
low-level networking stuff" rather than introducing people to
a set of packages working together where some piece of functionality
is here and some other piece is there.

Part of https://github.com/ooni/probe/issues/1591

internal/ooapi/swagger_test.go

@@ -1,5 +1,5 @@
 // Code generated by go generate; DO NOT EDIT.
-// 2021-09-05 13:54:22.884854 +0200 CEST m=+0.000556501
+// 2021-09-28 12:05:23.978512 +0200 CEST m=+0.000565751
 
 package ooapi
 
@@ -9,7 +9,7 @@ const swagger = `{
     "swagger": "2.0",
     "info": {
         "title": "OONI API specification",
-        "version": "0.20210905.9115422"
+        "version": "0.20210928.9100523"
     },
     "host": "api.ooni.io",
     "basePath": "/",