Buildable folders support

[pepicrft]

@bryansum

I don’t have a clear picture of what an API could be at this point, but I’m willing to iterate on a solution and come up with a proposal given there’s some interest / approval from you to do so.

This would be great. The two places where we’d need to add the API are:

• SourcesFileList
• ResourceFileElements

When envisioning the API, I’d recommend aligning with Xcode’s terminology. Maybe something along the lines of:

let target = Target(
  /..../
  sources: .synchronized("Sources/MyTarget"),
  resources: .synchronized("Resources/MyTarget"),
)

I’d start with the simplest API, and then iterate on that to add exceptions later.

Let me know if you need any guidance to contribute to the projects. I’d be happy to give you a rundown of the surfaces of the code bases that you’ll have to touch.