scripts/dl_github_archive.py: rename from download.py

 - Make the code more GitHub-specific
 - Requires mirror hash to work with .gitattributes
 - Use different API depending on whether PKG_SOURCE_VERSION is a
   complete commit id or other ref types like tags
 - Fix removing symbolic link
 - pre-clean dir_untar for possible leftovers from previous run

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>