[cloud_firestore] Rename back to firebase_firestore

[collinjackson]

The cloud_firestore package was originally named firebase_firestore. It was renamed cloud_firestore in response to an internal request (see flutter/flutter#12388). My understanding is that the Firebase team now prefers the name firebase_firestore so we might want to consider renaming the package back. This would require clients to change their pubspec.yaml so it should not be taken lightly, but if we're making a breaking change to 1.0 this might be the right time to do it.

This came up in the context of reviewing #1950 (https://github.com/FirebaseExtended/flutterfire/blob/ab87efff3fad051afdafff8dd172071fb9705041/packages/cloud_firestore/cloud_firestore_web/lib/cloud_firestore_web.dart) where I pushed back on the introduction of a new class, CloudFirestoreWeb.