• when use_ext is true, we will check to see if there are no supported
  modules for a particular file. instead of the file being skipped, an
  error will be thrown (RT #38927).
  
• also, when use_ext is true, a fatal error will be thrown if there are
  no loaders available that understand the file extension.
  
• officially support multiple loaders per extension
  
• add a Config::Any::Base for all loaders to inherit from, plus add
  a new dependency mechanism: requires_any_of() and requires_all_of().
  
• filter out loaders that don't inherit from Config::Any::Base (RT #40830)