hunter: disable external download when building with Hunter

The only package that is allowed to use `ocv_download()` is `IPPICV` as
it is tricked to use the Hunter provided file.

Author: NeroBurner

cmake/OpenCVDownload.cmake

@@ -86,6 +86,13 @@ endfunction()
 
 function(ocv_download)
   cmake_parse_arguments(DL "UNPACK;RELATIVE_URL" "FILENAME;HASH;DESTINATION_DIR;ID;STATUS" "URL" ${ARGN})
+  if(HUNTER_ENABLED)
+    if (DL_ID STREQUAL "IPPICV")
+      message(DEBUG "ocv_download(): IPPICV allowed while using Hunter, continue")
+    else()
+      message(FATAL_ERROR "ocv_download(): downloading external resources not allowed while bulding with Hunter. Args: ${ARGV}")
+    endif()
+  endif()
 
   function(ocv_download_log)
     file(APPEND "${OPENCV_DOWNLOAD_LOG}" "${ARGN}\n")