I have several emacs distributions (scimax, spacemacs, vanilla emacs, etc.) which all uses their own melpa directory. I want them all to use the same elpa (or melpa or gnu) directory to save some place.
For example, I have a "init.el" file in my vanilla emacs directory (~/.emacs.d/init.el). I want "Vanilla emacs" to look up elpa files inside in my scimax directory (~/emacs/scimax/.emacs.d/elpa) and download future elpa files inside the scimax's elpa directory.