Use private header import for FIRVersion.h (#7896)

* Use direct import for public FirebaseCore header.

This is an exception mentioned in https://github.com/firebase/firebase-ios-sdk/blob/master/HeadersImports.md#imports---for-header-file-consumers

* Fix import path.

The exception only applies to public headers *from other public headers*.

Author: ryanwilson

FirebaseRemoteConfig/Sources/RCNDevice.m

@@ -19,7 +19,7 @@
 #import <sys/utsname.h>
 
 #import <GoogleUtilities/GULAppEnvironmentUtil.h>
-#import "FirebaseCore/Sources/Public/FirebaseCore/FIRVersion.h"
+#import "FirebaseCore/Sources/Private/FirebaseCoreInternal.h"
 #import "FirebaseRemoteConfig/Sources/Private/RCNConfigSettings.h"
 #import "FirebaseRemoteConfig/Sources/RCNConfigConstants.h"