Support component plugins

[jkeifer]

Can we create a way to specify python packages that implement cirrus components as "plugins"? Rather than showing up as built-in or with a relative path in a project, they would display with the name of the plugin package defining them.

If we can work this out and it seems viable, we should split all nonessential components out of the builtins and put them in a separate package (or set of packages) so they don't clutter up projects that do not require them.

[jkeifer]

Plugin support is more or less implemented in #138, though supporting searching them for components will need a few more pieces put into place. Also we should document the plugin interface(s). Currently plugins can be registered via setuptools entrypoints, cirrus.plugins to point to the plugin package and cirrus.commands to add new commands to the cli.

We could easily add cirrus.components or something like that to point to a module containing feeders/functions/tasks/workflows dirs like cirrus.builtins, and we could have the component search automatically register those paths.

I suppose this should also support cloudformation resources as well, so maybe that entrypoint name is too specific.

[jkeifer]

Naming is hard. I am going with cirrus.resources for this before I change my mind yet again.