// swift-tools-version:4.0
import PackageDescription

let package = Package(
    name: "StringScanner",
    targets: [
        .target(
            name: "StringScanner",
            path: "Sources")
    ]
)

// swift-tools-version:4.0
import PackageDescription

let package = Package(
    name: "StringScanner"
)

--- 
+++ 
@@ -2,5 +2,10 @@
 import PackageDescription
 
 let package = Package(
-    name: "StringScanner"
+    name: "StringScanner",
+    targets: [
+        .target(
+            name: "StringScanner",
+            path: "Sources")
+    ]
 )
Add explicit target to package manifest
