~/.doom.d/modules is now a full module tree, like ~/.emacs.d/modules.
Symlinks are no longer involved.
Private modules can now shadow Doom modules. e.g.
~/.doom.d/modules/lang/org will take precendence over
~/.emacs.d/modules/lang/org.
Also, made doom--*-load-path variables public (e.g. doom--site-load-path
=> doom-site-load-path), and rearranged the load-path for a 10-15%
startup boost.