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/CMakeLists.txt

@@ -101,6 +101,7 @@ foreach(file ${files})
 
     target_compile_definitions(${testcase} PRIVATE CATCH_CONFIG_FAST_COMPILE)
     target_include_directories(${testcase} PRIVATE "thirdparty/catch")
+    target_include_directories(${testcase} PRIVATE ${NLOHMANN_JSON_INCLUDE_BUILD_DIR})
     target_link_libraries(${testcase} ${NLOHMANN_JSON_TARGET_NAME})
 
     if(NOT MSVC)