Django Evolution 2.1 is getting support for dependencies defined in

evolutions, allowing a single evolution or all evolutions for an app to

apply before or after other evolutions or migrations. These can be

specified as an app-level dependency (in the evolutions/__init__.py)

or an evolution-level dependency by setting the following:

BEFORE_EVOLUTIONS
BEFORE_MIGRATIONS
AFTER_EVOLUTIONS
AFTER_MIGRATIONS

Each is a list of items, where each item is either a string containing

an app label (for evolutions) or a tuple containing an app label and

evolution/migration name (for evolutions and migrations).
This change begins adding support for this by introducing utility

functions that will look up these dependencies.

get_evolution_app_dependencies() will fetch the dependencies specified

at the top-level evolutions/__init__.py for an app, and

get_evolution_dependencies() will fetch those defined in a particular

evolution.
Along with checking an evolution's specified dependencies,

get_evolution_dependencies() will also check if any mutations have

dependencies to inject. This is a new feature of mutations.
Currently only MoveToDjangoMigrations implements mutation-defined

dependencies, and ensures that the evolution will apply after the

migrations it applies to. This ensures that for a new database (or new

app), the migrations will always apply first (since the intent is to

ultimately have the app use them instead of evolutions). For an existing

app, the evolver will already make sure those migrations are marked as

applied by scanning ahead for this mutation, and it's important that the

evolution doesn't try to apply before this step happens.
Upcoming changes will factor the dependencies in when determining the

order in which migrations and evolutions will apply.

Unit tests pass.
Made use of this with upcoming changes to implement full dependency

support.