Rather than providing an already-resolved map of plugins to core, we now
provide a "provider resolver" which knows how to resolve a set of provider
dependencies, to be determined later, and produce that map.
This requires the context to be instantiated in a different way, so this
very noisy diff is a mostly-mechanical update of all of the existing
places where contexts get created for testing, using some adapted versions
of the pre-existing utilities for passing in mock providers.
c835ef8ff3