emacs: Add 'guix-devel-build-package-definition'.

Suggested by Ludovic Courtès <ludo@gnu.org>.

* emacs/guix-guile.el (guix-guile-definition-regexp): New variable.
  (guix-guile-current-definition, guix-guile-boolean): New functions.
* emacs/guix-devel.el: Require 'guix-base'.
  (guix-devel-repl-processes): New variable.
  (guix-devel-setup-repl, guix-devel-setup-repl-maybe): New functions.
  (guix-devel-build-package-definition): New command.
* doc/emacs.texi (Emacs Development): Document it.