Raise `PluginError` when the `url` attr is set and it doesn't match
any of the plugin's matchers, not just once upon class initialization.
Remove `None` type from `Plugin.matcher` and `Plugin.match`:
Since the input URL must always match, it is pointless to annotate
these attributes as potentially being `None`. This causes lots of
headaches in plugin implementations because it requires type-narrowing.
Initialize them with `None`, so plugin tests without matchers
don't have to call `hasattr()`.

Merged