[Setup] Non Apple platforms (#13)

This PR contains the work done to address the issue #12, with regards to provide basic support of non Apple platforms.

To provide further details about the work done:
- [x] flattened the folder structure, especially now that the idea to filter folders based on platform is being discarded;
- [x] implemented precompiler processors to filter out platform-specific source code;
- [x] updated the `Package` file to provide basic support for non-Apple platforms;
- [x] added and also improved some targets to the `Makefile` file to smooth the current development workflows;
- [x] updated the `.gitignore` file with references to the `.vscode` folder and the `.env` file;
- [x] updated the Swift tools version to v5.7.

Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Reviewed-on: #13

Tests/Dependencies/Cases/Services/DependencyServiceTests.swift

@@ -0,0 +1,61 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+import Dependencies
+import XCTest
+
+final class DependencyServiceTests: XCTestCase {
+
+    // MARK: Setup
+    
+    override func setUp() {
+        DependencyService[\.testService] = SomeService()
+    }
+    
+    // MARK: Tests
+
+    func test_readDependencyKey() async throws {
+        // GIVEN
+        // WHEN
+        let service = DependencyService[\.testService]
+        
+        // THEN
+        XCTAssertNotNil(service)
+        XCTAssert(service is SomeService)
+    }
+    
+    func test_writeDependencyKey() async throws {
+        // GIVEN
+        DependencyService[\.testService] = SomeOtherService()
+        
+        // WHEN
+        let service = DependencyService[\.testService]
+        
+        // THEN
+        XCTAssertNotNil(service)
+        XCTAssert(service is SomeOtherService)
+    }
+    
+    func test_writeDependencyKeyTwice() async throws {
+        // GIVEN
+        DependencyService[\.testService] = SomeOtherService()
+        DependencyService[\.testService] = SomeService()
+        
+        // WHEN
+        let service = DependencyService[\.testService]
+        
+        // THEN
+        XCTAssertNotNil(service)
+        XCTAssert(service is SomeService)
+    }
+
+}