error: 'native-ios-app': invalid custom path 'Artsy' for target 'eigen'

Hi,

Steps to reproduce:

1. Installing Danger through homebrew: `brew install danger/tap/danger-swift`
2. It creates `Dangerfile.swift` with the following content:

```
// swift-tools-version:4.2
import PackageDescription

let package = Package(
name: "Eigen",
dependencies: [
.package(url: "https://github.com/danger/swift.git", from: "1.0.0")
],
targets: [
// This is just an arbitrary Swift file in our app, that has
// no dependencies outside of Foundation, the dependencies section
// ensures that the library for Danger gets build also.
.target(name: "eigen", dependencies: ["Danger"], path: "Artsy", sources: ["Stringify.swift"]),
]
)
```
3. Running swift build `swift run danger-swift pr [url_of_pr]`, and getting the following error:
`error: 'native-ios-app': invalid custom path 'Artsy' for target 'eigen'`

Tried to install `Eigen` globally, but it does not help get rid of this error.