Carthage: Add support for Cartfile.private file

[tsteenbe]

In Carthage one can specify a Cartfile.private to define the project's build and test dependencies.

Propose extend Carthage.kt so any dependency found in Cartfile.private is assigned to the "private" scope in Ort so it's easy to exclude build and test frameworks. Maybe assign scope 'default' or 'public' to all dependencies found in Cartfile.resolved and not Cartfile.private?