plugin: Introduce HookRegistry.

This is the layer that keeps track of the different groups of hooks and
can filter them before runtime. The idea here is that you'll be able
to do things like skip hooks that require network access when you don't
have it, or skip CPU-intensive hooks when you don't need them, etc.

test_plugin_run: Simplify testing strategy.

Avoid keeping state in the hook classes/instances.

plugin: Rename the main method of hooks from `check` to `run`.

This will be more appropriate when we have hooks that do more than
check metadata.

plugin: Start entry point.

This doesn't integrate with existing hooks but at least sketches out the
main loop.