CopyRequest does not use atomic write

[JanTvrdik]

CopyRequest does not use atomic write, which may result in empty or corrupted zip filed in Composer cache directory.

CopyRequest should write the file to "$destination.tmp" and then rename("$destination.tmp", $destination) after the downloads succeeds.

You can find the corrupted files with

find ./files/ -type f -name '*.zip' -not -exec zip --quiet --test {} \;