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Issue Description:

If I have a SPM target with a custom path and the specified array of sources, prior to Swift 5.2 it was possible to add files outside of that path, like this:
.target(
{{ name:"DemoTarget",}}
{{ dependencies: [],}}
{{ path:"Foo",}}
{{ sources: [}}
{{ "Dummy.swift",}}
{{ "../Bar/FileOutsidePath.swift" // file in question}}
{{ ]}}
)

Now with Swift 5.2. this is not possible anymore and the file in question is not included in the target.

I pinned down the error in a minimal setup under https://github.com/rikner/c-swift-demo