Only expose `include/nlohmann/json.hpp` as public header in Bazel BUILD file

[thii]

All other headers are not meant to be included by a client of this library and should be declared in srcs.

This uses https://github.com/bazelbuild/rules_swift/blob/40c36c936c9c80b4aefa3f008ecf99dbe002be2c/third_party/com_github_nlohmann_json/BUILD.overlay as a reference.

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Pull request checklist

Read the Contribution Guidelines for detailed information.

• [x] Changes are described in the pull request, or an existing issue is referenced.
• [ ] The test suite compiles and runs without error.
• [ ] Code coverage is 100%. Test cases can be added by editing the test suite.
• [x] The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header files single_include/nlohmann/json.hpp and single_include/nlohmann/json_fwd.hpp. The whole process is described here.

[coveralls]

Coverage: 100.0%. Remained the same when pulling 7cb526e4da4892d68a0938383849e1bb26ff2857 on thii:thi/only-expose-include-nlohmann-json.hpp-as-public-header into 546370c9e778d99e7176641123e5cc1d0b62acab on nlohmann:develop.

[gregmarr]

All other headers are not meant to be included by a client of this library.

That is not an accurate statement in general. The single include is simply an option for those users that want a "simpler" install.

[thii]

Got it. Thanks.