Delegation

[joguSD]

Implemented Wizard Delegation. Wizard specifications can now specify a Retrieval step to delegate like so:

"Retrieval": {
    "Type": "Wizard",
    "Resource": "SubWizardName",
    "Path": "JMESPath.query"
}

A stage's resolution is no longer a void operation, and the data that would be stored in the environment is now also returned at the end of the stage. Upon executing the final stage the return value from it is returned as the final result of executing the wizard.

This PR also has a couple fixes to old unit tests to increase coverage.

[joguSD]

Unit tests are currently failing after an upstream change on the cli. The root cause is that pytest will capture IO by default and stubs input so that it will throw exceptions if it's attempted to be read. I've opened an upstream pull request on pytest to add the missing attribute to their stub.
pytest-dev/pytest#1866

[kyleknap]

This is a little confusing here especially since a delegation loader could be itself. The docstring may be needed to be updated here. I am not super familiar with the architecture but I am assuming you add the delegation loader if you delegate to a different wizard so if None is provided wouldn't that mean there is no wizard to delegate to?

[joguSD]

I made the delegation_loader an init parameter to facilitate unit testing. Initially, the loader that was passed was always self which meant that if a wizard would delegate it would always attempt to load the wizard from disk. This allows me to pass a stub/mock for the delegation loader. Using self as the delegation_loader is intended to be the default behavior. I can improve the docstring for this.

[kyleknap]

Interesting. Usually it is not the best practice to be adding an extra parameter solely for unit testing. Can you see the parameter ever being used in the future? If not, is there any other way of testing this like could just use the Stage interface directly since the wizard loader gets injected there?

[joguSD]

Fixed this up. Rather than inject a WizardLoader, I simply register a custom data loader on the session and inject that session instead.

[kyleknap]

Looks good. Just had a question on the interface.

[kyleknap]

Cool. Interface looks much better. 🚢