bug#57503: 28.1.91; package-selected-packages should not be saved to custom-file

[Stefan Kangas]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> Hm...  I don't immediately see any way to make this work seamlessly in a
> way that's backwards/forwards compatible.

How about something like this:

- Keep the old defcustom, and add a new multisession variable.

- If the old variable exists, use it.  Otherwise the new one.

- Add a new command that "upgrades" the variable into a multisession
  variable.  Make it say that it will break things on Emacs < 29.  Don't
  run it automatically.

- In Emacs version N (e.g., 5 major versions from now?), upgrade any
  user automatically, without prompting, and make the defcustom
  obsolete.