Use the SDK version from Firebase package.json

This change is needed since the firebase-exp SDK is not meant to depend on @firebase/app

Author: schmidt-sebastian

packages/firestore/src/core/version.ts

@@ -15,7 +15,5 @@
  * limitations under the License.
  */
 
-import firebase from '@firebase/app';
-
 /** The semver (www.semver.org) version of the SDK. */
-export const SDK_VERSION = firebase.SDK_VERSION;
+export { version as SDK_VERSION } from '../../../firebase/package.json';