Avoid breaking highlighting after adding new font-lock keywords.
* emacs/guix-base.el (guix-insert-package-strings): Use 'propertize' instead
of 'guix-get-string'.
* emacs/guix-info.el (guix, guix-action, guix-file, guix-url,
guix-package-location, guix-package-name): New button types.
(guix-info-insert-action-button, guix-info-insert-file-path,
guix-info-insert-url, guix-package-info-insert-location,
guix-package-info-insert-full-names,
guix-package-info-insert-non-unique-text): Adjust for 'guix-insert-button'
and button types.
(guix-package-info-name-button): New face.
(guix-package-info-define-insert-inputs): Use it. Add new button types.
(guix-package-info-insert-full-name): Remove.
* emacs/guix-utils.el (guix-get-string): Replace 'face' with 'font-lock-face'.
(guix-insert-button): Adjust for using button types.