Move BuildExtensions into the Components folder

Complete grouping the standalone modules under Components. Updates the local
Swift package path in project.yml.

Components/BuildExtensions/Sources/MacrosImplementation/Plugin.swift

@@ -0,0 +1,14 @@
+//
+// Copyright 2026 Element Creations Ltd.
+//
+// SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+// Please see LICENSE files in the repository root for full details.
+//
+
+import SwiftCompilerPlugin
+import SwiftSyntaxMacros
+
+@main
+struct BuildExtensionsPlugin: CompilerPlugin {
+    let providingMacros: [Macro.Type] = [UserPreferenceMacro.self, AppHookMacro.self]
+}