Refactor and modernize namespace declarations

 * Completely remove inline namespace v2
 * Flatten some unnecessarily deep namespace nesting
 * Move DummyClientFace, Segmenter, SegmentFetcher to namespace ndn
 * Move all unit tests to namespace ndn::tests

Change-Id: I8bcfcf9fd669936a3277d2d5d505f765b4b05742
diff --git a/tools/ndnsec/util.cpp b/tools/ndnsec/util.cpp
index ae407db..a85b9e3 100644
--- a/tools/ndnsec/util.cpp
+++ b/tools/ndnsec/util.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2013-2022 Regents of the University of California.
+ * Copyright (c) 2013-2023 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -25,8 +25,7 @@
 
 #include <unistd.h>
 
-namespace ndn {
-namespace ndnsec {
+namespace ndn::ndnsec {
 
 bool
 getPassword(std::string& password, const std::string& prompt, bool shouldConfirm)
@@ -89,5 +88,4 @@
   NDN_CXX_UNREACHABLE;
 }
 
-} // namespace ndnsec
-} // namespace ndn
+} // namespace ndn::ndnsec