Make backdrop{,.collector} namespace packages

This is needed so that other packages can provide things in this namespace.

All packages must have `extend_path` in them, otherwise whether this works or
not will depend on the fluke of which order they happen to be visited in the
path.

There will be an analogous change to backdrop-collector-plugins to
use pkgutil too, since namespace_package is a setuptools thing and
pkgutil is builtin to python.

/cc @robyoung