emacs: Link subdirs.el into emacs-packages-deps

Emacs loads all the elisp files in the top-level of the site-lisp
directory. However some packages (e.g. mu4e) put their elisp files in a
subdirectory of site-lisp. Emacs will not load these packages unless
subdirs.el is present.

This commit links the subdirs.el file from the emacs package into the
emacs-package-deps package so that packages that put their elisp files
in a subdirectory of site-lisp are loaded.