Implement an internal plugin model

[ehmatthes]

Supporting #313, implement an internal plugin model. Just make simple_deploy.py call out to a deploy() function in each platform's module?

• [x] Move all platform-specific code to subdirs in simple_deploy/management/commands/platforms/.
• [x] Use pluggy to call the platform deployer from simple_deploy.
  • [x] fly
  • [x] platform_sh
  • [x] heroku
• [ ] Current tests pass.
• [ ] Merge.
• [ ] Move tests to platform-specific subdirs.
• [ ] Update tests.

[ehmatthes]

Move all platform-specific code to subdirs

• [x] What code is outside these dirs currently? Is it anything more than tests?
  • [x] developer_resources

[ehmatthes]

Use pluggy to call the platform deployer

• [ ] Function to get bool for whether automate_all is supported, and confirm_automate_all message.
• [x] Start by calling a hook implementation that returns a confirm_automate_all msg.
  • [x] No errors, but not getting the message. Only getting an empty list; should get a list from the hook caller, containing any return values from hook impl functions. (I was importing the wrong module, so there was no hook implementation provided by the plugin.)
• [x] Function to manage rest of deployer process.
  • [x] Move instantiation of the PlatformDeployer object to the deploy module.
  • [x] Integration tests pass for Fly.
  • [x] e2e test passes for Fly.

[ehmatthes]

External plugin notes

• [ ] Update logic around validating platform name, and message if not supported. Maybe you forgot to install a plugin?
• [ ] External plugins will need to be able to modify the output of --help.
• [ ] How does naming work if there's an open ecosystem for deployment plugins? For example, someone offers a different approach to deploying to fly? I think people need to choose names different than what's included in core, and then it's on the user to not install two plugins with the same name? Maybe if we find more than one, we ask the user which one to use?

[ehmatthes]

Closed by #327.