Formula.factory: raise on encountering non-Formula

Formula.factory naively assumed that any already-loaded constant
fed into it was a formula, with confusing results when trying to
`brew install` a name from the ruby stdlib.

Fixes Homebrew/homebrew#16284.

Library/Homebrew/formula.rb

@@ -395,6 +395,8 @@ class Formula
       raise LoadError
     end
 
+    raise NameError if klass.superclass != Formula
+
     return klass.new(name) if install_type == :from_name
     return klass.new(name, path.to_s)
   rescue NoMethodError