* lisp/emacs-lisp/package.el: Fix a decoding issue

* lisp/emacs-lisp/package.el: Fix a decoding issue

(package--with-response-buffer): Use `url-insert-buffer-contents'.
The previous code had some issues with decoding. Refactoring that
function allows us to use the decoding from url-handlers while still
treating both sync and async requests the same.

* lisp/url/url-handlers.el (url-insert-file-contents): Move some code to
`url-insert-buffer-contents'.
(url-insert-buffer-contents): New function