Rewrite includes inside the library to use <catch2/> form

src/catch2/catch_config.cpp

@@ -5,9 +5,9 @@
  *  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
  */
 
-#include "catch_config.hpp"
-#include "catch_enforce.h"
-#include "catch_stringref.h"
+#include <catch2/catch_config.hpp>
+#include <catch2/catch_enforce.h>
+#include <catch2/catch_stringref.h>
 
 namespace Catch {