ui: Report profile hooks separately.

ui: Report profile hooks separately.

* guix/ui.scm (profile-hook-derivation?): New procedure.
(show-what-to-build): Distinguish among BUILD derivations that match
'profile-hook-derivation?'.  Report them separately.
* guix/status.scm (hook-message): New procedure.
(print-build-event): Display profile hooks with readable hook name.
* guix/profiles.scm (info-dir-file, ghc-package-cache-file,
ca-certificate-bundle, glib-schemas, gtk-icon-themes, gtk-im-modules,
xdg-desktop-database, xdg-mime-database, fonts-dir-file, manual-database):
Augment derivation with "type" and "hook" properties.