Add basic swift pm support

Package.swift

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+// swift-tools-version:4.0
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import PackageDescription
+
+let package = Package(
+    name: "Cryptex",
+    products: [
+        // Products define the executables and libraries produced by a package, and make them visible to other packages.
+        .library(
+            name: "Cryptex",
+            targets: ["Cryptex"]),
+    ],
+    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: "Cryptex",
+            dependencies: []),
+        .testTarget(
+            name: "CryptexTests",
+            dependencies: ["Cryptex"]),
+    ]
+)

Sources/Cryptex/Cryptex.swift

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+struct Cryptex {
+    var text = "Hello, World!"
+}

Tests/CryptexTests/CryptexTests.swift

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+import XCTest
+@testable import Cryptex
+
+class CryptexTests: XCTestCase {
+    func testExample() {
+        // This is an example of a functional test case.
+        // Use XCTAssert and related functions to verify your tests produce the correct
+        // results.
+        XCTAssertEqual(Cryptex().text, "Hello, World!")
+    }
+
+
+    static var allTests = [
+        ("testExample", testExample),
+    ]
+}

Tests/LinuxMain.swift

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+import XCTest
+@testable import CryptexTests
+
+XCTMain([
+    testCase(CryptexTests.allTests),
+])