get_evolution_dependencies() can now be passed a list of custom

evolutions, which will be searched if a module for the evolution cannot

be found. This is in the same format that EvolveAppTask takes.

Dependencies can be specified within each custom evolution or through

listed mutations (such as MoveToDjangoMigrations).
This will soon be used by the evolver, primarily for the purpose of unit

testing.

Unit tests pass.
Tested this along with pending changes to the evolver.