refactor(netx): move construction logic outside package (#798)

For testability, replace most if-based construction logic with
calls to well-tested factories living in other packages.

While there, acknowledge that a bunch of types could now be private
and make them private, modifying the code to call the public
factories allowing to construct said types instead.

Part of https://github.com/ooni/probe/issues/2121

internal/bytecounter/counter_test.go

@@ -2,7 +2,7 @@ package bytecounter
 
 import "testing"
 
-func TestGood(t *testing.T) {
+func TestCounter(t *testing.T) {
 	counter := New()
 	counter.CountBytesReceived(16384)
 	counter.CountKibiBytesReceived(10)