Renamed the Sources folder as Libraries.

Libraries/Dependency/Protocols/DependencyKey.swift

@@ -0,0 +1,26 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftLibs open source project
+//
+// Copyright (c) 2023 Röck+Cöde VoF. and the SwiftLibs project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftLibs project authors
+//
+//===----------------------------------------------------------------------===//
+
+/// This protocol defines a key to use in the dependency service.
+public protocol DependencyKey {
+    
+    // MARK: Associated types
+    
+    /// The associated type representing the type of the dependency key's value.
+    associatedtype Value
+    
+    // MARK: Properties
+    
+    /// The default value for the dependency key.
+    static var currentValue: Value { get set }
+    
+}