Hi,

in the recent discussion on reserving a keymap for packages, I proposed
extending package.el to support a sort of formal specification of what a
user should or could customize. As there were some supportive comments,
I attempted to improve on an earlier proof-of-concept[0], resulting in
the attached patch.

This introduces the following changes:

- User option `package-query-suggestions', to enable or disable these
  suggestions. I have disabled this feature by default, because it might
  be annoying. It is probably better for template-configurations or a
  theme to enable it.

- Variable pacakge-configuration-suggestions, that packages add their
  suggestions to. Here's an example how this could look like for avy:

    ;;;###autoload
    (add-to-list 'pacakge-configuration-suggestions
		 `(avy (key "Avy's entry-point are commands like avy-goto-char\
    that have to be bound globally"
			    ,(kbd "C-:")
			    avy-goto-char)))

  Beside keys, one can currently also specify options and hook. It might
  be worth distinguishing between options and global minor-modes.

- Function package-suggest-configuration, that generates the
  configuration. It is automatically called by package-install, but can
  also be invoked manually.

There are a few things I am not satisfied with, such as that the default
behaviour for package-suggest-configuration is to just append the
generated configuration to `custom-file' or `user-init-file'. Part of my
intention was to generate code that can easily be changed and adapted by
the user (unlike custom-set-variables), so I don't analyse the files
themselves. This might not look nice in some cases, but then again,
these people are probably not the ones using this feature

Another point is that package-suggest-configuration has an option such
that the command will not change anything (PREVIEW, activated with a
prefix argument). I was wondering if it would make sense to make this
the default behaviour whenever the command is invoked interactively.

[0] https://lists.gnu.org/archive/html/help-gnu-emacs/2021-02/msg00305.html

Interested in your comments,
           Philip K.