factory: rescue NameError

Due to the new const_defined? checks, passing certain names (such as
those that start with a digit) that are illegal constant names will
raise NameError. Catch this and error out gracefully, as we would have
previously.

Fixes Homebrew/homebrew#14342.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/formula.rb

@@ -403,7 +403,7 @@ class Formula
 
     return klass.new(name) if install_type == :from_name
     return klass.new(name, target_file)
-  rescue LoadError
+  rescue LoadError, NameError
     raise FormulaUnavailableError.new(name)
   end