keg: add ALL_TOP_LEVEL_DIRECTORIES constant.

This is used in diagnostic (and will be used in other places later)
rather than hard-coding other directories later.

Library/Homebrew/diagnostic.rb

@@ -308,8 +308,7 @@ module Homebrew
       def check_access_prefix_directories
         not_writable_dirs = []
 
-        extra_dirs = ["lib/pkgconfig", "share/locale", "share/man", "opt"]
-        (Keg::TOP_LEVEL_DIRECTORIES + extra_dirs).each do |dir|
+        Keg::ALL_TOP_LEVEL_DIRECTORIES.each do |dir|
           path = HOMEBREW_PREFIX/dir
           next unless path.exist?
           next if path.writable_real?

Library/Homebrew/keg.rb

@@ -65,6 +65,7 @@ class Keg
   LOCALEDIR_RX = /(locale|man)\/([a-z]{2}|C|POSIX)(_[A-Z]{2})?(\.[a-zA-Z\-0-9]+(@.+)?)?/
   INFOFILE_RX = %r{info/([^.].*?\.info|dir)$}
   TOP_LEVEL_DIRECTORIES = %w[bin etc include lib sbin share var Frameworks]
+  ALL_TOP_LEVEL_DIRECTORIES = (TOP_LEVEL_DIRECTORIES + %w[lib/pkgconfig share/locale share/man opt]).freeze
   PRUNEABLE_DIRECTORIES = %w[bin etc include lib sbin share Frameworks LinkedKegs].map do |d|
     case d when "LinkedKegs" then HOMEBREW_LIBRARY/d else HOMEBREW_PREFIX/d end
   end