WIP: Use find_package for nlohmann_json

[ghost]

This begins the work of #2802 (comment)

Fixes #2874

This PR, whenever possible, attempts to install nlohmann_json via the package manager. If it is unavailable, then we install it from source. Note that since different distros package different versions, then it might be a good idea to install a newer version from source. However, the disadvantage of this is that many package maintainers would prefer the packaged version, even if it is old, because it will contain patches and packaging standards relevant to that distro. On the other side of the argument, supporting a version as old as 2.1.1 would be absurd.

I have only tested one Dockerfile manually on my system; testing all of it would be time consuming. I'm relying on the first round of Kokoro builds to spot any errors. I apologize in advance.

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This change is 

[coryan]

I have only tested one Dockerfile manually on my system; testing all of it would be time consuming. I'm relying on the first round of Kokoro builds to spot any errors. I apologize in advance.

No need to apologize, that is why we have robots to do this work. If you do get errors please test your fixes locally first though.

[coryan]

In general the changes look great. I have a question, but I have not thought about this much: I really do not know what the answer is.

Reviewed 34 of 34 files at r1.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @remyabel)

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google/cloud/storage/CMakeLists.txt, line 47 at r1 (raw file):

endif ()

find_package(nlohmann_json CONFIG REQUIRED)

I think that means if anybody installs nlohmann_json without installing the CMake dependencies the find_package() would fail. Can we just say find_package(nlohmann_json REQUIRED)?

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google/cloud/storage/CMakeLists.txt, line 338 at r1 (raw file):

        internal/logging_resumable_upload_session_test.cc
        internal/metadata_parser_test.cc
        internal/nljson_use_after_third_party_test.cc

Can we preserve these tests?

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google/cloud/storage/internal/nljson.h, line 43 at r1 (raw file):

#define nlohmann google_cloud_storage_internal_nlohmann_3_4_0
#include "google/cloud/storage/internal/nlohmann_json.hpp"

I am not sure, but maybe we want to preserve this? Obviously we need to change this line to #include <nlohmann/json.hpp> but maybe we can preserve the stuff around it?

[ghost]

Hmm, this will be less straight forward than I thought. I'm running into strange compilation errors:

In file included from /home/build/google-cloud-cpp/google/cloud/storage/internal/nljson_use_after_third_party_test.cc:23:
/home/build/google-cloud-cpp/google/cloud/storage/internal/nljson.h:70:26: error: variable or field 'PrintTo' declared void
   70 | inline void PrintTo(json const& j, std::ostream* os) { *os << j.dump(); }
      |                          ^~~~~
/home/build/google-cloud-cpp/google/cloud/storage/internal/nljson.h:70:21: error: 'json' was not declared in this scope
   70 | inline void PrintTo(json const& j, std::ostream* os) { *os << j.dump(); }
      |                     ^~~~
/home/build/google-cloud-cpp/google/cloud/storage/internal/nljson.h:70:21: note: suggested alternatives:
In file included from /home/build/google-cloud-cpp/google/cloud/storage/internal/nljson_use_after_third_party_test.cc:20:
/usr/include/json.hpp:871:7: note:   'nlohmann::json'
  871 | using json = basic_json<>;
      |       ^~~~
In file included from /home/build/google-cloud-cpp/google/cloud/storage/internal/nljson_use_after_third_party_test.cc:20:
/usr/include/json.hpp:2259:29: note:   'nlohmann::detail::input_format_t::json'
 2259 | enum class input_format_t { json, cbor, msgpack, ubjson, bson };
      |                             ^~~~
In file included from /home/build/google-cloud-cpp/google/cloud/storage/internal/nljson_use_after_third_party_test.cc:23:
/home/build/google-cloud-cpp/google/cloud/storage/internal/nljson.h:70:48: error: expected primary-expression before '*' token
   70 | inline void PrintTo(json const& j, std::ostream* os) { *os << j.dump(); }
      |                                                ^
/home/build/google-cloud-cpp/google/cloud/storage/internal/nljson.h:70:50: error: 'os' was not declared in this scope; did you mean 'cos'?
   70 | inline void PrintTo(json const& j, std::ostream* os) { *os << j.dump(); }
      |                                                  ^~
      |                                                  cos

I think this might be a compatibility issue with 3.6.1.

[ghost]

I am closing this for now. There are a surprising amount of obstacles here:

Step 55/81 : RUN cmake -H. -Bcmake-out
 ---> Running in 55baf1f8a701
CMake Error at CMakeLists.txt:1 (cmake_minimum_required):
  CMake 3.8 or higher is required.  You are running version 3.5.1