More flexible additional packages

When the package already exists, use configuration-layer/make-package
to copy information into the existing package object.

This allows overwriting location, step and excluded, but NOT owner,
pre-layers or post-layers.

This also adds a new valid location attribute called `local-dotfile`.
This is an implementation detail, to distinguish local packages (in
layers) from packages which have their location overridden to local.
This is based on the assumption that the source code in the latter case
should not be found in a subdirectory of the owning layer, but somewhere
else entirely. For those packages, `.emacs.d/local/<pkg-name>` will be
added to the load path.

core/core-configuration-layer.el

@@ -102,7 +102,7 @@
    (location :initarg :location
              :initform elpa
              :type (satisfies (lambda (x)
-                                (or (member x '(local elpa))
+                                (or (member x '(local local-dotfile elpa))
                                     (and (listp x) (eq 'recipe (car x))))))
              :documentation "Location of the package.")
    (step :initarg :step
@@ -146,7 +146,7 @@ directory with a name starting with `+'.")
   (configuration-layer//configure-layers configuration-layer--layers)
   ;; packages
   (setq configuration-layer--packages (configuration-layer//declare-packages
-                                      configuration-layer--layers))
+                                       configuration-layer--layers))
   (setq configuration-layer--used-distant-packages
         (configuration-layer//get-distant-used-packages
          configuration-layer--packages))
@@ -329,11 +329,17 @@ Properties that can be copied are `:location', `:step' and `:excluded'."
     ;; additional and excluded packages from dotfile
     (when dotfile
       (dolist (apkg dotspacemacs-additional-packages)
-        (let ((obj (object-assoc apkg :name result)))
-          (unless obj
+        (let* ((apkg-name (if (listp apkg) (car apkg) apkg))
+               (props (when (listp apkg) (cdr apkg)))
+               (obj (object-assoc apkg-name :name result)))
+          (if obj
+              (configuration-layer/make-package apkg obj)
             (setq obj (configuration-layer/make-package apkg))
-            (push obj result))
-          (oset obj :owner 'dotfile)))
+            (push obj result)
+            (oset obj :owner 'dotfile))
+          ;; If location is specified and equal to local, make it local-dotfile
+          (when (eq 'local (plist-get props :location))
+            (oset obj :location 'local-dotfile))))
       (dolist (xpkg dotspacemacs-excluded-packages)
         (let ((obj (object-assoc xpkg :name result)))
           (unless obj
@@ -358,7 +364,7 @@ Properties that can be copied are `:location', `:step' and `:excluded'."
   "Return the distant packages (ie to be intalled) that are effectively used."
   (configuration-layer/filter-objects
    packages (lambda (x) (and (not (null (oref x :owner)))
-                             (not (eq 'local (oref x :location)))
+                             (not (memq (oref x :location) '(local local-dotfile)))
                              (not (oref x :excluded))))))
 
 (defun configuration-layer//get-private-layer-dir (name)
@@ -747,6 +753,11 @@ path."
           ;; TODO remove extensions in 0.105.0
           (push (format "%sextensions/%S/" dir pkg-name) load-path))
         (configuration-layer//configure-package pkg))
+       ((eq 'local-dotfile (oref pkg :location))
+        (push (format "%slocal/%S/" user-emacs-directory pkg-name) load-path)
+        ;; TODO remove extensions in 0.105.0
+        (push (format "%sextensions/%S/" user-emacs-directory pkg-name) load-path)
+        (configuration-layer//configure-package pkg))
        (t
         (configuration-layer//activate-package pkg-name)
         (configuration-layer//configure-package pkg))))))