[Firestore] On SPM, import FirebaseFirestoreInternalWrapper (#14848)

Author: ncooke3

Firestore/Swift/Source/SwiftAPI/Stages.swift

@@ -14,9 +14,14 @@
  * limitations under the License.
  */
 
-import FirebaseFirestoreInternal
 import Foundation
 
+#if SWIFT_PACKAGE
+  @_exported import FirebaseFirestoreInternalWrapper
+#else
+  @_exported import FirebaseFirestoreInternal
+#endif // SWIFT_PACKAGE
+
 protocol Stage {
   var name: String { get }
   var bridge: StageBridge { get }