fix cmake install directory (for real this time)

* Rename 'develop' folder to 'include/nlohmann'
* Rename 'src' folder to 'single_include/nlohmann'
* Use <nlohmann/*> headers in sources and tests
* Change amalgamate config file

test/src/fuzzer-parse_json.cpp

@@ -21,7 +21,7 @@ Licensed under the MIT License <http://opensource.org/licenses/MIT>.
 
 #include <iostream>
 #include <sstream>
-#include <json.hpp>
+#include <nlohmann/json.hpp>
 
 using json = nlohmann::json;