Address implementationOnly warnings

Author: ncooke3

FirebaseStorage/Sources/Internal/StorageTokenAuthorizer.swift

@@ -17,7 +17,7 @@ import Foundation
 import FirebaseAppCheckInterop
 import FirebaseAuthInterop
 import FirebaseCore
-@_implementationOnly import FirebaseCoreExtension
+internal import FirebaseCoreExtension
 
 #if COCOAPODS
   import GTMSessionFetcher

FirebaseStorage/Sources/Storage.swift

@@ -19,7 +19,7 @@ import FirebaseAuthInterop
 import FirebaseCore
 
 // Avoids exposing internal FirebaseCore APIs to Swift users.
-@_implementationOnly import FirebaseCoreExtension
+internal import FirebaseCoreExtension
 
 /// Firebase Storage is a service that supports uploading and downloading binary objects,
 /// such as images, videos, and other files to Google Cloud Storage. Instances of `Storage`

FirebaseStorage/Sources/StorageUploadTask.swift

@@ -15,9 +15,9 @@
 import Foundation
 
 #if COCOAPODS
-  @_implementationOnly import GoogleUtilities
+  internal import GoogleUtilities
 #else
-  @_implementationOnly import GoogleUtilities_Environment
+  internal import GoogleUtilities_Environment
 #endif // COCOAPODS
 
 #if COCOAPODS