dependency: don't try and expand yourself.

This can happen on e.g. fossil where the dependency HEAD requires having
an install of fossil in order to check it out. Bit of a silly edge case
but this handles it.

Fixes Homebrew/homebrew#25605

Library/Homebrew/dependency.rb

@@ -85,6 +85,7 @@ class Dependency
         when :keep_but_prune_recursive_deps
           [dep]
         else
+          next [] if dependent.to_s == dep.name
           expand(dep.to_formula, &block) << dep
         end
       end.flatten