bug#81508: 31.0.50; package.el archive contents download robustness issues

X-debbugs-cc: Philip Kaludercic <[email protected]>

If there is an ENOSPC in the middle of writing the archive-contents
file, the truncated file is left behind.  This is because
package--download-one-archive uses plain write-region instead of writing
into a temporary file and then atomically renaming it into place.

Worse, if a truncated file has been left behind,
package--read-archive-file errors instead of trying to delete and
re-download the file.

-- 
Sean Whitton