emacs: move notmuch-emacs-mua under emacs

While the notmuch-emacs-mua script is a sort of cli command, it is
really a part of notmuch-emacs. Move it under the emacs directory.

diff --git a/notmuch-emacs-mua b/emacs/notmuch-emacs-mua
similarity index 100%
rename from notmuch-emacs-mua
rename to emacs/notmuch-emacs-mua