is duplicated by:

• SR-1868 SwiftPM should avoid building unneeded dependency modules

relates to:

• SR-883 Support conditional dependencies

Issue Description:

Lets take the following project as an example: https://github.com/czechboy0/Jay/blob/master/Package.swift

In this project, theres the library that a project can use, and then an example application. As someone who is using Jay as a dependency, I don't care to compile JayExample when I run swift build. Jay should be able to specify which files, folders, or targets to exclude when being built as a dependency. However, if I'm working on Jay, or checking it out, and I'm in a checkout of Jay, I wouldn't want to exclude JayExample from building (which is why the existing exclude doesn't work; if ALWAYS excludes those files). I would imagine the Package file for Jay should look something like this:

import PackageDescription

let package = Package(
    name: "Jay",
    exclude: [],
    excludeWhenDependant: ["Sources/JayExample"],
    targets: [
        Target(
            name: "Jay"
        ),
        Target(
            name: "JayExample",
            dependencies: [
                .Target(name: "Jay")
            ]
        )
    ]
)