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/mocks/dnsencoder.go

@@ -0,0 +1,11 @@
+package mocks
+
+// DNSEncoder allows mocking dnsx.DNSEncoder.
+type DNSEncoder struct {
+	MockEncode func(domain string, qtype uint16, padding bool) ([]byte, error)
+}
+
+// Encode calls MockEncode.
+func (e *DNSEncoder) Encode(domain string, qtype uint16, padding bool) ([]byte, error) {
+	return e.MockEncode(domain, qtype, padding)
+}