WireGuardKit: Add swift package scaffolding

Signed-off-by: Andrej Mihajlov <and@mullvad.net>

WireGuardKit/Sources/WireGuardKit/WireGuardKit.swift

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+// SPDX-License-Identifier: MIT
+// Copyright © 2018-2019 WireGuard LLC. All Rights Reserved.
+
+import Foundation
+import libwg_go
+
+public func getWireGuardVersion() -> String {
+    return String(cString: wgVersion()!)
+}

WireGuardKit/Sources/libwg-go/dummy.c

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+// Empty

WireGuardKit/Sources/libwg-go/include/libwg_go.h

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+#include "../../../../wireguard-go-bridge/wireguard.h"

WireGuardKit/Sources/libwg-go/module.modulemap

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+module libwg_go {
+    umbrella header "include/libwg_go.h"
+    link "wg-go"
+    export *
+}