Pass config and backend to plugins

They should have references to the config and the backend, but not to
the whole Spicedham object.
Update tests to pass a config and a backend
Instatiate plugins with a config and a backend
A file to import for the side effects

Gross.
Change from entry points to subclass lookup

Recursively look up subclasses. Note that we also import a file for import
side effects. That's gross.