Add Package.swift

From protocolbuffers#7052

Package.swift

@@ -0,0 +1,39 @@
+// swift-tools-version:4.1
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+import PackageDescription
+
+let package = Package(
+    name: "Protobuf",
+    products: [
+        // Products define the executables and libraries produced by a package, and make them visible to other packages.
+        .library(name: "Protobuf", targets: ["Protobuf"]),
+    ],
+    dependencies: [
+        // Dependencies declare other packages that this package depends on.
+        // .package(url: /* package url */, from: "1.0.0"),
+    ],
+    targets: [
+        // Targets are the basic building blocks of a package. A target can define a module or a test suite.
+        // Targets can depend on other targets in this package, and on products in packages which this package depends on.
+        .target(
+            name: "Protobuf",
+            dependencies: [],
+            path: "objectivec",
+            exclude: [
+                // The following would cause duplicate symbol definitions.GPBProtocolBuffers is expected to be
+                // left out, as it's an umbrella implementation file.
+                "GPBProtocolBuffers.m",
+                "DevTools/",
+                "Tests/",
+                "BUILD.bazel",
+                "generate_well_known_types.sh",
+                "ProtocolBuffers_OSX.xcodeproj",
+                "ProtocolBuffers_iOS.xcodeproj",
+                "ProtocolBuffers_tvOS.xcodeproj",
+                "README.md",
+             ],
+            sources: ["."]
+//            publicHeadersPath: "."
+        ),
+    ]
+)