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/fakeapi_test.go

@@ -1,5 +1,5 @@
 // Code generated by go generate; DO NOT EDIT.
-// 2021-09-05 13:54:21.412036 +0200 CEST m=+0.000169751
+// 2021-09-28 12:05:22.411961 +0200 CEST m=+0.000187918
 
 package ooapi