There is no point spamming a list for every function call, there is no
use case where multiple notify callbacks would be useful.

First step to making it a fatal error.

If no ADD_ROUTE message has been received from the master associating a
stream with a particular context ID, then it is expected messages
originating from that context ID can only be routed via the parent.

inspect module includes the staticmethod decorator added by
a8d1dc67304d678e9e94ebb64be259e958181b63

This version is based on the modulefinder standard library module,
pruned back just to handle modules we know have been loaded already, and
to scan module-level imports only, rather than imports occurring in
class and function scope (crappy heuristic, but assume they are lazy
imports).

The ast and compiler modules were far too slow, whereas this version can
bytecode compile and scan all the imports for django.db.models (58
modules) in around 200ms.. 3.4ms per dependency, it's probably not going
to get much faster than that.