formula: reject dot dirs as racks

Dot directories are not racks because formulae names do not start with
"." so avoid attempting to descend into dot directories, which may not
always be possible (e.g. .Trashes).

Library/Homebrew/formula.rb

@@ -1371,7 +1371,7 @@ class Formula
   def self.racks
     @racks ||= if HOMEBREW_CELLAR.directory?
       HOMEBREW_CELLAR.subdirs.reject do |rack|
-        rack.symlink? || rack.subdirs.empty?
+        rack.symlink? || rack.basename.to_s.start_with?(".") || rack.subdirs.empty?
       end
     else
       []