Prune LinkedKegs relative to correct path

Fixes Homebrew/homebrew#16948.

Library/Homebrew/cmd/doctor.rb

@@ -202,7 +202,7 @@ end
 def check_for_broken_symlinks
   require 'keg'
   broken_symlinks = []
-  Keg::PRUNEABLE_DIRECTORIES.map { |d| HOMEBREW_PREFIX/d }.each do |d|
+  Keg::PRUNEABLE_DIRECTORIES.each do |d|
     next unless d.directory?
     d.find do |pn|
       broken_symlinks << pn if pn.symlink? and pn.readlink.expand_path.to_s =~ /^#{HOMEBREW_PREFIX}/ and not pn.exist?

Library/Homebrew/cmd/prune.rb

@@ -9,7 +9,7 @@ module Homebrew extend self
     $d = 0
     dirs = []
 
-    Keg::PRUNEABLE_DIRECTORIES.map{ |d| HOMEBREW_PREFIX/d }.each do |path|
+    Keg::PRUNEABLE_DIRECTORIES.each do |path|
       next unless path.directory?
       path.find do |path|
         path.extend ObserverPathnameExtension

Library/Homebrew/keg.rb

@@ -11,7 +11,9 @@ class Keg < Pathname
   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]
-  PRUNEABLE_DIRECTORIES = %w[bin etc include lib sbin share Frameworks Library/LinkedKegs]
+  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
 
   # if path is a file in a keg then this will return the containing Keg object
   def self.for path