Replace @_implementationOnly with internal

Author: andrewheard

FirebaseVertexAI/Sources/VertexAI.swift

@@ -18,7 +18,7 @@ import FirebaseCore
 import Foundation
 
 // Avoids exposing internal FirebaseCore APIs to Swift users.
-@_implementationOnly import FirebaseCoreExtension
+internal import FirebaseCoreExtension
 
 /// The Vertex AI for Firebase SDK provides access to Gemini models directly from your app.
 @available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)

FirebaseVertexAI/Sources/VertexLog.swift

@@ -15,7 +15,7 @@
 import Foundation
 import os.log
 
-@_implementationOnly import FirebaseCoreExtension
+internal import FirebaseCoreExtension
 
 enum VertexLog {
   /// Log message codes for the Vertex AI SDK