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

[Lars Ingebrigtsen]

Philip Kaludercic <philipk@posteo.net> writes:

>> Hm...  I don't immediately see any way to make this work seamlessly in a
>> way that's backwards/forwards compatible.
>
> What about if we allowed setting `package-selected-packages' to a symbol
> like 'external, in which case the list would be saved in a file?  That
> way users would have to explicitly opt-in.

But that would still break things for people who go back to using an
older Emacs version.