[6.0][PackageDescription] Deprecated swiftLanguageVersion should result … (#7826)

…in `swiftLanguageMode` setting

- Explanation:

Otherwise manifest loading is going to fail because
`swiftLanguageVersion` has been removed.

- Main Branch PR:
#7825

- Resolves:
#7823
- Resolves: rdar://132484168

- Risk: Very Low (fixes a bug).

- Reviewed By: @MaxDesiatov 

- Testing: New tests were added.

(cherry picked from commit d1405f2)

Author: xedin

Sources/PackageDescription/BuildSettings.swift

@@ -402,7 +402,7 @@ public struct SwiftSetting: Sendable {
       _ condition: BuildSettingCondition? = nil
     ) -> SwiftSetting {
         return SwiftSetting(
-            name: "swiftLanguageVersion", value: [.init(describing: version)], condition: condition)
+            name: "swiftLanguageMode", value: [.init(describing: version)], condition: condition)
     }
 
     /// Defines a `-language-mode` to pass  to the

Tests/PackageLoadingTests/PD_6_0_LoadingTests.swift

@@ -66,6 +66,40 @@ final class PackageDescription6_0LoadingTests: PackageDescriptionLoadingTests {
         }
     }
 
+    func testSwiftLanguageModesPerTarget() async throws {
+        try UserToolchain.default.skipUnlessAtLeastSwift6()
+
+        let content = """
+                import PackageDescription
+                let package = Package(
+                    name: "Foo",
+                    defaultLocalization: "fr",
+                    products: [],
+                    targets: [
+                        .target(
+                            name: "Foo",
+                            swiftSettings: [
+                                .swiftLanguageMode(.v5)
+                            ]
+                        ),
+                        .target(
+                            name: "Bar",
+                            swiftSettings: [
+                                .swiftLanguageVersion(.v6)
+                            ]
+                        )
+                    ]
+                )
+                """
+
+        let observability = ObservabilitySystem.makeForTesting()
+        let (_, validationDiagnostics) = try await loadAndValidateManifest(content, observabilityScope: observability.topScope)
+        XCTAssertNoDiagnostics(validationDiagnostics)
+        testDiagnostics(observability.diagnostics) { result in
+            result.checkUnordered(diagnostic: .contains("'swiftLanguageVersion' is deprecated: renamed to 'swiftLanguageMode(_:_:)'"), severity: .warning)
+        }
+    }
+
     private func loadRootManifestWithBasicGitRepository(
         manifestContent: String, 
         validator: (Manifest, TestingObservability) throws -> ()

Tests/WorkspaceTests/ManifestSourceGenerationTests.swift

@@ -589,7 +589,7 @@ final class ManifestSourceGenerationTests: XCTestCase {
         try await testManifestWritingRoundTrip(manifestContents: contents, toolsVersion: .v5_9)
     }
 
-    func testManifestGenerationWithSwiftLanguageVersion() async throws {
+    func testManifestGenerationWithSwiftLanguageMode() async throws {
         try UserToolchain.default.skipUnlessAtLeastSwift6()
         let manifest = Manifest.createRootManifest(
             displayName: "pkg",