Lazily registering hooks?

[Vincent Ladeuil]

A couple of related ideas in addition of the ones already mentioned in
this thread:

- the main problem with hook (and command !) registrations is the
  requirement to import the module,
- the main achievement of lazy loading is that the module import is
  delayed until needed.

To achieve the same result with hooks, some centralization is required
such as the bzrlib.hooks.known_hooks one (but nothing more).

If we add the indication that a hook is fully loaded, it becomes
possible to check for it before firing this hook.