Detect a non-list package archive content properly (Bug#22311)

Detect a non-list package archive content properly (Bug#22311)

* lisp/emacs-lisp/package.el (package--download-one-archive): Use
`read' instead of `read-from-string'; the latter always returns a
cons, so the `listp' check on its return value doesn't make sense.  It
was changed from `read' to `read-from-string' in 2015-04-01 "*
emacs-lisp/package.el: Implement asynchronous refreshing", but that
change was not needed because `read' works fine on strings as well as
buffers.