firebase-ios-sdk

Fix and rewrite sync_project.rb (#3296)

* Fix and rewrite sync_project.rb

Ever since we added multiple platforms, removing a file has been broken.
This fixes that issue.

Also, rewrite target synchronization to make more sense. Essentially now
there's one diff between the filesystem and the groups and then an
explicit diff is done between project files and target contents.

* Run sync_project.rb on post-install