If `'loaders'` is present in the `TEMPLATES` options together with
`APP_DIRS` set to `True`, the template engine raises an exception. This
conflict is now detected by the system check templates.E001.
This is the result of Christopher Medrela's 2013 Summer of Code project.
Thanks also to Preston Holmes, Tim Graham, Anssi Kääriäinen, Florian
Apolloner, and Alex Gaynor for review notes along the way.
Also: Fixes#8579, fixes#3055, fixes#19844.