`doom-module-load-path` will be removed in the next big refactor commit,
so as a stop gap, I simply move it to doom.el to resolve the reference
error.
This also removes the obsolete alias `doom-modules-dirs`, since it's
been deprecated long enough (and `doom-module-load-path` itself won't be
around much longer anyway).
Ref: #8147
Amend: 8cafbe4408
e1a2f94ec1