This patch adds the following to 'package-install-selected-packages':
- a call to 'package--archives-initialize', to initialize the archive
contents, similar to how 'package-install' works;
- a new optional argument NOCONFIRM, to don't ask for confirmation when
installing new packages.

Thread on emacs-devel:
https://lists.gnu.org/archive/html/emacs-devel/2021-03/msg00662.html