[mercury] Rename layer: mercury-layer to mercury

Thanks Miciah for the suggestion.

CHANGELOG.develop

@@ -381,6 +381,7 @@ sane way, here is the complete list of changed key bindings
 *** Renamed layers
 - =extra-langs= renamed to =major-modes= (thanks to Eivind Fonn)
 - =pdf-tools= renamed to =pdf=
+- =mercury-layer= renamed to =mercury=
 *** Removed layers
 - evil-cleverparens (moved to =spacemacs-evil=)
 - extra-langs (replaced by =ad-hoc= layers)

layers/+lang/mercury/packages.el

@@ -9,7 +9,7 @@
 ;;
 ;;; License: GPLv3
 
-(defconst mercury-layer-packages
+(defconst mercury-packages
   '((metal-mercury-mode :location (recipe
                                    :fetcher github
                                    :repo "ahungry/metal-mercury-mode"