Major rewrite of doom module API

+ Fix #446, where the .local/packages.el cache was generated with
  a faulty load-path.
+ Entries in the doom-modules hash table are now plists, containing
  :flags and :path, at least.
+ Add doom-initialize-modules for loading module config.el files.
+ Add doom-module-get for accessing this plist, e.g.

    (doom-module-get :some module)         ; returns plist
    (doom-module-get :some module :flags)  ; return specific property

+ Replace doom-module-enable with doom-module-set, e.g.

    (doom-module-set :some module :flags '(+a +b +c))

+ Remove doom-module-flags (use doom-module-get instead)
+ Rename doom-module-enabled-p with doom-module-p
+ Replace doom-module-path with doom-module-find-path and
  doom-module-expand-file. The former will search for an existing module
  or file in doom-modules-dirs. The latter will expand the path from
  whatever path is stored in doom-modules.
+ Replace doom-module-paths with doom-module-load-path
+ Changed doom! to allow for nested doom! calls by delaying the loading
  of module config.el files until as late as possible.
+ Refactor doom-initialize-packages to only ihitialize package state
  (i.e. doom-packages, package-alist, and quelpa-cache), rather than its
  previous behavior of loading all Doom files (and sometimes all module
  files). This is faster and more predictable.

core/autoload/help.el

@@ -95,9 +95,9 @@ in, or d) the module associated with the current major mode (see
                             nil t module))))
   (cl-destructuring-bind (category submodule)
       (mapcar #'intern (split-string module " "))
-    (unless (doom-module-enabled-p category submodule)
+    (unless (doom-module-p category submodule)
       (error "'%s' isn't a valid module" module))
-    (let ((doc-path (expand-file-name "README.org" (doom-module-path category submodule))))
+    (let ((doc-path (doom-module-expand-file category submodule "README.org")))
       (unless (file-exists-p doc-path)
         (error "There is no documentation for this module"))
       (find-file doc-path))))