Conditionally install os packages #741

Fixes an issue where exec-shell-from-path could not be installed on
Linux or Windows.

Doom used :ignore because, at the time, it supported a workflow where
your Emacs config was shared over dropbox or rslsync across multiple
computers. This workflow is no longer supported (it was very buggy!), so
this is no longer necessary. :ignore should also be reserved for private
use and not used internally.

core/packages.el

@@ -2,8 +2,9 @@
 ;;; core/packages.el
 
 ;; core-os.el
-(package! exec-path-from-shell :ignore (not IS-MAC))
-(package! osx-clipboard        :ignore (not IS-MAC))
+(when IS-MAC
+  (package! exec-path-from-shell)
+  (package! osx-clipboard))
 
 ;; core-ui.el
 (package! all-the-icons)