Updated Package.swift to SPM v4 spec. Updated directory layout for SPM v4.

Author: tcldr

Package.resolved

@@ -1,6 +1,13 @@
-import PackageDescription
+// swift-tools-version:4.0
 
-let package = Package(name: "Operadics")
+import PackageDescription
 
-let operadics = Product(name: "Operadics", type: .Library(.Dynamic), modules: "Operadics")
-products.append(operadics)
+let package = Package(
+    name: "Operadics",
+    products: [
+        .library(name: "Operadics", targets: ["Operadics"]),
+    ],
+    targets: [
+        .target(name: "Operadics", dependencies: [])
+    ]
+)