using same go type for infrastructure and applications, misc 🧹

[WillBeebe]

• added methods IsInfrastructure() and IsApplication to the bundle
• removed pure functions that did this checking in the cmd package
• replaced those functions with calls to the new methods
• removed the Application type
• updated references to the shared type

It's backwards-compat and prints a warning if the bundle type is bundle.

[WillBeebe]

blocked by: https://github.com/massdriver-cloud/massdriver-cli/pull/148