Refactor folder (#10475)

Move many third-party components' source codes from the src folder to a new folder called deps_src. The goal is to make the code structure clearer and easier to navigate.

deps_src/nlohmann/detail/iterators/iterator_traits.hpp

@@ -0,0 +1,51 @@
+#pragma once
+
+#include <iterator> // random_access_iterator_tag
+
+#include <nlohmann/detail/meta/void_t.hpp>
+#include <nlohmann/detail/meta/cpp_future.hpp>
+
+namespace nlohmann
+{
+namespace detail
+{
+template<typename It, typename = void>
+struct iterator_types {};
+
+template<typename It>
+struct iterator_types <
+    It,
+    void_t<typename It::difference_type, typename It::value_type, typename It::pointer,
+    typename It::reference, typename It::iterator_category >>
+{
+    using difference_type = typename It::difference_type;
+    using value_type = typename It::value_type;
+    using pointer = typename It::pointer;
+    using reference = typename It::reference;
+    using iterator_category = typename It::iterator_category;
+};
+
+// This is required as some compilers implement std::iterator_traits in a way that
+// doesn't work with SFINAE. See https://github.com/nlohmann/json/issues/1341.
+template<typename T, typename = void>
+struct iterator_traits
+{
+};
+
+template<typename T>
+struct iterator_traits < T, enable_if_t < !std::is_pointer<T>::value >>
+            : iterator_types<T>
+{
+};
+
+template<typename T>
+struct iterator_traits<T*, enable_if_t<std::is_object<T>::value>>
+{
+    using iterator_category = std::random_access_iterator_tag;
+    using value_type = T;
+    using difference_type = ptrdiff_t;
+    using pointer = T*;
+    using reference = T&;
+};
+} // namespace detail
+} // namespace nlohmann