This makes several updates to support the blockupdates plugin.
I had to update several hooks that were using the wrong types, and
provide a way to get event.Feed objects into plugins without importing
event.Feed (which I did by having the plugin loader make them
available).
When the plugin loader itself had to know the types in the arguments
and return values of the plugin functions, it was very difficult to
avoid import loops, given that the types were often defined in the
same package that needed to invoke the plugins.
Under this model, the plugin loader has much less knowledge of the
plugins themselves, and within each package we define functions to
interact with the plugins.
