[xcodegen] Avoid buildable folders for mixed Swift targets

If a Swift target has sources in its folder that
aren't part of the target, we can't form a buildable
folder.

utils/swift-xcodegen/Sources/SwiftXcodeGen/Path/PathProtocol.swift

@@ -127,6 +127,10 @@ extension PathProtocol {
     hasExtension(.c, .cpp, .m, .mm)
   }
 
+  var isSourceLike: Bool {
+    isCSourceLike || hasExtension(.swift)
+  }
+
   var isDocLike: Bool {
     hasExtension(.md, .rst) || fileName.starts(with: "README")
   }