Improve default bundle handling for SPM and Xcode targets

[kublaios]

Our current implementation assumes the consumer is using Xcode project test targets when trying to find the feature files. The defaults don’t work for an SPM test target, you need to remember to pass .module or it won’t find the features. Given the way things are going, it doesn't make sense to have such a default.

A naive solution would involve two changes:

• For the Feature() function, we make bundle a required param. For Xcode projects, the consumer would specify Bundle(for: Self.self) and for SPM .module.
• GenerateFeature_EXPERIMENTAL also has bundle as a required param. When generating the code, it should preserve the bundle param from its call site in the generated Feature call.

We can also see if there's a cleaner solution that avoids the user having to pass the bundle explicitly.