Prevent source taint, minimize json dep size

Added the OpenCV code to prevent corrupting src and polluting repo from here:
https://github.com/opencv/opencv/blob/c96f48e7c91748f896e99f5bf943faf08cd98f0e/CMakeLists.txt#L12

Switched to the JSON cmake specific repo that has a lot of improvements

CMakeLists.txt

@@ -1,3 +1,12 @@
+# Disable in-source builds to prevent source tree corruption.
+if(" ${CMAKE_SOURCE_DIR}" STREQUAL " ${CMAKE_BINARY_DIR}")
+  message(FATAL_ERROR "
+FATAL: In-source builds are not allowed.
+       You should create a separate directory for build files.
+")
+endif()
+
+
 set(CMAKE_CXX_COMPILER g++)
 set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_C_COMPILER gcc)

deps/json/CMakeLists.txt

@@ -1,4 +1,10 @@
 include(FetchContent)
 
-FetchContent_Declare(json URL https://github.com/nlohmann/json/releases/download/v3.11.2/json.tar.xz)
+# https://github.com/ArthurSonzogni/nlohmann_json_cmake_fetchcontent
+FetchContent_Declare(json
+  GIT_REPOSITORY https://github.com/ArthurSonzogni/nlohmann_json_cmake_fetchcontent
+  GIT_PROGRESS TRUE
+  GIT_SHALLOW TRUE
+  GIT_TAG v3.11.2)
+
 FetchContent_MakeAvailable(json)