Allow to use quelpa-use-package as source of truth for quelpa-cache

[kiennq]

This allows the quelpa-cache to be populated from the usage of quelpa-use-package only.
Since we're using quelpa-cache as a way to indicate which packages are managed by quelpa, this change allows those packages are correctly populated via the config files only.

[alphapapa]

I've left some comments on individual lines. Beyond those, please write a bit about the purpose of this PR and why it's necessary, to help me understand it.

[alphapapa]

This form belongs in the defcustom's setter so it will take effect when the option is changed.

[alphapapa]

It may be unclear what "this" refers to.

As well, it seems unclear what "managed by Quelpa" means.

[alphapapa]

Should the parent group be use-package or quelpa?

[alphapapa]

It's not harmful to specify the prefix, but it's unnecessary for options specified later in the same file.

[alphapapa]

The first line of a docstring should be a complete sentence. If this one's is being updated, it might as well be fixed.

[alphapapa]

The name of this variable seems unclear, and a bit mysterious. Could it be given a more descriptive one?

[kiennq]

I've left some comments on individual lines. Beyond those, please write a bit about the purpose of this PR and why it's necessary, to help me understand it.

Thanks, let me clarify the purpose of this PR before addressing your comments.
Currently, quelpa is marking which packages it's managed via quelpa-cache, which is also used to store the custom recipes as well. The cache will be updated when we install a new package via quelpa, or get restored from the cache file if we specify the quelpa-persistent-cache-p.

So, if we don't install any new package with quelpa in this Emacs session and don't have quelpa-persistent-cache-p on, running quelpa-upgrade-all will do nothing (besides upgrading quelpa).
On other hand, if we have unnecessary packages in the quelpa-cache, doing upgrades will cost a lot of time since we're doing repo upgrades for all of those packages as well.

Since we have this :quelpa syntax with use-package, I think we can consider all packages that are declared with :quelpa in the config file as implicitly an entry in the quelpa-cache.
Using that and turn off quelpa-persistent-cache-p will make us have a clean quelpa-cache every time Emacs starts, with the exact information as specified in the config file.