For a usual profile, packages are placed in a profile directory itself,
but for a system profile, packages are placed in 'profile'
sub-directory. So we need to do some special cases for system profiles
to find packages there as well.
* emacs/guix-base.el (guix-packages-profile): New procedure.
(guix-manifest-file): Use it. Add optional 'system?' argument.
* emacs/guix-ui-generation.el (guix-system-generation?)
(guix-generation-current-packages-profile): New procedures.
(guix-generation-packages, guix-generation-insert-packages): Remove
'generation' argument.
(guix-generation-packages-buffer): Add optional 'system?' argument.
(guix-profile-generation-manifest-file)
(guix-profile-generation-packages-buffer): Adjust accordingly.
* emacs/guix-main.scm (generation-package-specifications+paths): Rename to...
(profile->specifications+paths): ... this. Use a single 'profile' argument.
* emacs/guix-ui.el (guix-ui-read-profile): New procedure.
* emacs/guix-ui-package.el (guix-search-by-name)
(guix-search-by-regexp, guix-installed-packages)
(guix-obsolete-packages, guix-all-available-packages)
(guix-newest-available-packages): Use it.
* emacs/guix-ui-generation.el (guix-generations)
(guix-last-generations, guix-generations-by-time): Use it.
* emacs/guix-buffer.el (guix-define-groups, guix-define-entry-type)
(guix-define-buffer-type): New macros.
(guix-buffer-define-interface): Add parent groups for the generated
custom groups.
* emacs/guix-info.el: Use 'guix-define-buffer-type' to generate custom
groups.
* emacs/guix-list.el: Likewise.
* emacs/guix-ui.el: Use 'guix-define-groups' to generate custom groups.
(guix-ui-define-entry-type): New macro.
* emacs/guix-ui-package.el: Use it.
* emacs/guix-ui-generation.el: Use it.
