caveats: Differentiate zsh completion files and function files

When installing a file to zsh/site-functions directory, it is assumed this is a zsh completion file,
and the zsh completion caveat is printed after installation.

But not all files in the zsh/site-functions directory are completion files.
Some are files for functions that can be loaded on demand with zsh's autoload command.

- Edit Keg.completion_installed to search specifically for files in the zsh/site-functions
  directory starting with an underscore only (By convention, zsh completion files start with an underscore)
- Add Keg.zsh_functions_installed to search for non-completion files in the zsh/site-functions
- Add Caveats.zsh_function_caveats to print a caveat if non-completion files have been installed
  to zsh/site-functions

Library/Homebrew/caveats.rb

@@ -19,6 +19,7 @@ class Caveats
     caveats << bash_completion_caveats
     caveats << zsh_completion_caveats
     caveats << fish_completion_caveats
+    caveats << zsh_function_caveats
     caveats << fish_function_caveats
     caveats << plist_caveats
     caveats << python_caveats
@@ -100,6 +101,16 @@ class Caveats
     EOS
   end
 
+  def zsh_function_caveats
+    return unless keg
+    return unless keg.zsh_functions_installed?
+
+    <<-EOS.undent
+      zsh functions have been installed to:
+        #{HOMEBREW_PREFIX}/share/zsh/site-functions
+    EOS
+  end
+
   def fish_function_caveats
     return unless keg
     return unless keg.fish_functions_installed?

Library/Homebrew/keg.rb

@@ -297,12 +297,21 @@ class Keg
   def completion_installed?(shell)
     dir = case shell
     when :bash then path.join("etc", "bash_completion.d")
-    when :zsh  then path.join("share", "zsh", "site-functions")
+    when :zsh
+      dir = path.join("share", "zsh", "site-functions")
+      dir if dir && dir.directory? && dir.children.any? { |f| f.basename.to_s.start_with?("_") }
     when :fish then path.join("share", "fish", "vendor_completions.d")
     end
     dir && dir.directory? && !dir.children.empty?
   end
 
+  def zsh_functions_installed?
+    # Check for non completion functions (i.e. files not started with an underscore),
+    # since those can be checked separately
+    dir = path.join("share", "zsh", "site-functions")
+    dir && dir.directory? && dir.children.any? { |f| !f.basename.to_s.start_with?("_") }
+  end
+
   def fish_functions_installed?
     dir = path.join("share", "fish", "vendor_functions.d")
     dir && dir.directory? && !dir.children.empty?