Add bzrlib.pyutils, which has get_named_object, a wrapper around __import__.
This is used to replace various ad hoc implementations of the same logic, notably the version used in registry's _LazyObjectGetter which had a bug when getting a module without also getting a member. And of course, this new function has unit tests, unlike the replaced code.
This also adds a KnownHooksRegistry subclass to provide a more natural home for some other logic.
I'm not thrilled about the name of the new module or the new functions, but it's hard to think of good names for such generic functionality.