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/netxlite/errorsx/errno_windows.go

@@ -1,37 +0,0 @@
-// Code generated by go generate; DO NOT EDIT.
-// Generated: 2021-09-27 15:57:58.455744 +0200 CEST m=+0.259427834
-
-package errorsx
-
-import "golang.org/x/sys/windows"
-
-const (
-	ECANCELED       = windows.ECANCELED
-	ECONNREFUSED    = windows.ECONNREFUSED
-	ECONNRESET      = windows.ECONNRESET
-	EHOSTUNREACH    = windows.EHOSTUNREACH
-	ETIMEDOUT       = windows.ETIMEDOUT
-	EAFNOSUPPORT    = windows.EAFNOSUPPORT
-	EADDRINUSE      = windows.EADDRINUSE
-	EADDRNOTAVAIL   = windows.EADDRNOTAVAIL
-	EISCONN         = windows.EISCONN
-	EFAULT          = windows.EFAULT
-	EBADF           = windows.EBADF
-	ECONNABORTED    = windows.ECONNABORTED
-	EALREADY        = windows.EALREADY
-	EDESTADDRREQ    = windows.EDESTADDRREQ
-	EINTR           = windows.EINTR
-	EINVAL          = windows.EINVAL
-	EMSGSIZE        = windows.EMSGSIZE
-	ENETDOWN        = windows.ENETDOWN
-	ENETRESET       = windows.ENETRESET
-	ENETUNREACH     = windows.ENETUNREACH
-	ENOBUFS         = windows.ENOBUFS
-	ENOPROTOOPT     = windows.ENOPROTOOPT
-	ENOTSOCK        = windows.ENOTSOCK
-	ENOTCONN        = windows.ENOTCONN
-	EWOULDBLOCK     = windows.EWOULDBLOCK
-	EACCES          = windows.EACCES
-	EPROTONOSUPPORT = windows.EPROTONOSUPPORT
-	EPROTOTYPE      = windows.EPROTOTYPE
-)