emacs: Add 'guix-package-from-file' command.

* emacs/guix-main.scm (register-package, packages-from-file): New procedures.
(%patterns-makers): Add 'from-file' search type.
* emacs/guix-messages.el (guix-messages): Add messages for it.
* emacs/guix-ui-package.el (guix-package-from-file): New command.
(guix-package-info-insert-location): Adjust for 'from-file' type.
* doc/emacs.texi (Emacs Commands): Document it.