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/dnsx/dnsx.go

@@ -0,0 +1,14 @@
+// Package dnsx contains the dnsx model.
+package dnsx
+
+// HTTPSSvc is an HTTPSSvc reply.
+type HTTPSSvc struct {
+	// ALPN contains the ALPNs inside the HTTPS reply
+	ALPN []string
+
+	// IPv4 contains the IPv4 hints.
+	IPv4 []string
+
+	// IPv6 contains the IPv6 hints.
+	IPv6 []string
+}