Change header include directory to nlohmann/json

[patrikhuber]

Hi!

The readme advises to include the header with #include "json.hpp". If we add this repo as a git submodule, we would add the src/ directory of this repo to the include path, and also do #include "json.hpp". However, if one runs the CMake INSTALL target of this repo, or uses it via e.g. vcpkg, the header gets copied to the location nlohmann/json.hpp and should now be included as #include "nlohmann/json.hpp".
I think it would be better (and best practice) to put the header in the repo into a directory include/nlohmann/. Most header-only libraries that I know of do this. This would unify the #include: We could just use #include "nlohmann/json.hpp, no matter whether we're including via git submodule, CMake INSTALL target, vcpkg, etc. Another plus is that it sort-of "namespaces" the header, i.e. a clash with a potential other json.hpp file in any of the compiler's include directories is avoided (not unlikely, as json.hpp is quite a generic name).

[nlohmann]

I am afraid that changing the path of the single header from src/json.hpp to anything else would also break existing systems.

[nlohmann]

Ok. This will be fixed with #944.

[patrikhuber]

Great to see this awesome discussion on that matter! The PR will make me very happy as well ;-) 👍

[nlohmann]

Fixed with #944.