This introduces two new classes: DependencyGraph and EvolutionGraph.
DependencyGraph is a generic class for managing dependencies between

nodes and returning an order using a topological sort. Each node

stores a key, an insertion order (the order in which it was added

to the graph), a dictionary of state, and lists of nodes that it depends

on or that depend on it.
Dependencies are lazily-added through the graph, and can be removed.

When all manipulations of the graph are complete, a caller will call

finalize() to record dependencies, validate the graph, and allow

fetching the sorted order of nodes.
The previously-mentioned insertion order is an important implementation

detail in this graph. These work as a sort of loose dependency between

nodes. If two nodes don't define explicit dependencies to specify their

order, the insertion order will be used. This helps keep the order

generally consistent with the evolution ordering that we've used in the

past.
EvolutionGraph specializes this to manage dependencies

between evolutions, migrations, and model creations. An entire list of

evolutions and model creations for an app can be added at once, or an

entire migration plan. The graph will take care of determining the

correct dependencies for these and apply them. It then allows the caller

to retrieve batches of evolutions/migrations/model creations, grouped by

type and in the order indicated by dependencies.
As of this change, the graph isn't used outside of unit tests. Upcoming

changes will switch Evolver over to using the graph to apply

evolutions and migrations in the correct order.

Unit tests pass on all supported versions of Python and Django.