Use + instead of ! for layer categories

Helm seems to treat "!" specially in pattern matching, so having a ! in
the pattern string when traversing directories is problematic. This
change fixes #2737, because as far as I can tell "+" has no special
meaning in a helm pattern.

Of course, we can choose a different character, but I'm fond of "+" as
representing "more layers here".

core/core-configuration-layer.el

@@ -134,7 +134,7 @@ the path for this layer.")
 
 (defvar configuration-layer-categories '()
   "List of strings corresponding to category names. A category is a
-directory with a name starting with `!'.")
+directory with a name starting with `+'.")
 
 (defun configuration-layer/sync ()
   "Synchronize declared layers in dotfile with spacemacs."
@@ -390,7 +390,7 @@ Possible return values:
   nil      - the directory is a regular directory."
   (when (file-directory-p path)
     (if (string-match
-         "^!" (file-name-nondirectory
+         "^+" (file-name-nondirectory
                (directory-file-name
                 (concat configuration-layer-directory path))))
         'category
@@ -405,14 +405,14 @@ Possible return values:
 
 (defun configuration-layer//get-category-from-path (dirpath)
   "Return a category symbol from the given DIRPATH.
-The directory name must start with `!'.
+The directory name must start with `+'.
 Returns nil if the directory is not a category."
   (when (file-directory-p dirpath)
     (let ((dirname (file-name-nondirectory
                     (directory-file-name
                      (concat configuration-layer-directory
                              dirpath)))))
-      (when (string-match "^!" dirname)
+      (when (string-match "^+" dirname)
         (intern (substring dirname 1))))))
 
 (defun configuration-layer//discover-layers ()
@@ -479,7 +479,7 @@ path."
   (dolist (layer dotspacemacs-configuration-layers)
     (let ((layer-name (if (listp layer) (car layer) layer)))
       (if (ht-contains? configuration-layer-paths layer-name)
-          (unless (string-match-p "!distribution"
+          (unless (string-match-p "+distribution"
                                   (ht-get configuration-layer-paths layer-name))
             (push (configuration-layer/make-layer layer)
                   configuration-layer--layers))

core/core-dotspacemacs.el

@@ -54,7 +54,7 @@ exists. Otherwise, fallback to ~/.spacemacs")
 
 (defvar dotspacemacs-distribution 'spacemacs
   "Base distribution to use. This is a layer contained in the directory
-`!distribution'. For now available distributions are `spacemacs-core'
+`+distribution'. For now available distributions are `spacemacs-core'
 or `spacemacs'.")
 
 (defvar dotspacemacs-configuration-layer-path '()

core/core-themes-support.el

@@ -185,7 +185,7 @@ package name does not match theme name + `-theme' suffix.")
           (eq 'solarized-dark theme))
       (add-to-list 'load-path
                    (concat configuration-layer-directory
-                           "!distribution/spacemacs/local/solarized-theme/"))
+                           "+distribution/spacemacs/local/solarized-theme/"))
       (require 'solarized)
       (deftheme solarized-dark
         "The dark variant of the Solarized colour theme")

doc/CONTRIBUTE.org

@@ -48,10 +48,10 @@ chatroom]] and ask your questions there. It will be a pleasure to help you to
 contribute!
 
 ** Submitting a configuration layer
-Contributed configuration layers are stored in the =contrib= folder. The
-=contrib= folder also contains categories prefixed with =!= to put your
+Contributed configuration layers are stored in the =layers= folder. The
+=layers= folder also contains categories prefixed with =+= to put your
 layers in. For example a layer for a language would go in the
-=contrib/!lang= folder.
+=layers/+lang= folder.
 
 It is recommended to join a =README.org= file with your layer:
   - ideally this file should document the packages of your layer as well as the