cmake: update ocv_download

- more aggressive cache for files under CMAKE_BINARY_CACHE
  * don't re-read files MD5
  * don't repack archives
- add support for ENV{OPENCV_DOWNLOAD_PATH}
- added ID parameter as project/group identifier
- non-flat .cache directory (based on ID)
- download message prefix based on ID
- more detailed logging via ocv_download_log() macro
- force .gitignore file for .cache folder (with '*' pattern)

3rdparty/ffmpeg/ffmpeg.cmake

@@ -25,6 +25,7 @@ function(download_win_ffmpeg script_var)
                  "${OPENCV_FFMPEG_URL}"
                  "https://raw.githubusercontent.com/opencv/opencv_3rdparty/${FFMPEG_BINARIES_COMMIT}/ffmpeg/"
                DESTINATION_DIR ${FFMPEG_DOWNLOAD_DIR}
+               ID FFMPEG
                RELATIVE_URL
                STATUS res)
     if(NOT res)