security: Rename security/v2/* to security/*

Security v1-v2 transition has been completed for a while and path
separation is no longer needed. However, this commit preserves
`ndn::security::v2` namespace, but makes it inline.

Some commonly used v2 headers are preserved, but are deprecated and
should be avoided in the dependent software.

Change-Id: I26056cb3d34bcdc0a16e06dea8c1a78c03da2d40
diff --git a/tests/unit/security/verification-helpers.t.cpp b/tests/unit/security/verification-helpers.t.cpp
index c0c6b5a..dd5a832 100644
--- a/tests/unit/security/verification-helpers.t.cpp
+++ b/tests/unit/security/verification-helpers.t.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2013-2019 Regents of the University of California.
+ * Copyright (c) 2013-2020 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -466,7 +466,7 @@
   BOOST_CHECK(!verifySignature(unsignedInterest1, invalidKey, sizeof(invalidKey)));
 
   // - base version of verifySignature is tested transitively
-  // - pib::Key version is tested as part of v2/key-chain.t.cpp (Security/V2/TestKeyChain)
+  // - pib::Key version is tested as part of key-chain.t.cpp (Security/TestKeyChain)
 }
 
 BOOST_FIXTURE_TEST_CASE(VerifyHmac, IdentityManagementFixture)