Define `helm-source-buffers-list' at run time.

* helm-buffers.el (helm-source-buffers-list): Now nil.
(helm-buffers-list): Build source here.

* helm-files.el: Build buffer source in helm-for-files.

* helm-misc.el:  Build buffer source in helm-mini.
Fix prompt in lisp completion.

* helm-elisp.el (helm-lisp-completion-at-point): Do it.