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

src/catch2/catch_decomposer.cpp

@@ -6,8 +6,8 @@
  *  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
  */
 
-#include "catch_decomposer.h"
-#include "catch_config.hpp"
+#include <catch2/catch_decomposer.h>
+#include <catch2/catch_config.hpp>
 
 namespace Catch {