[NeedleInAJayStack]

This adds a command plugin that can be run like:

swift package --allow-writing-to-package-directory generate-code-from-graphql

To support this, the existing build plugin was refactored to share as much code as possible. I took pretty heavy inspiration from swift-openapi-generator's plugin code when implementing this.

Closes #17

[NeedleInAJayStack]

This work raises some additional configuration considerations. Off the top of my head:

• We don't currently allow configuring access modifiers (public, private, etc) on the generated types. They are all generated as implicit internal.
• The command plugin just puts files into the root of Sources. Should they be organized?

For reference here are the swift-openapi-generator configuration options