graphite/txstatsd: re-introduce require_package

We need require_package as the two classes have overlapping package
needs; however when using require_package, the resource is not always
declared within the same module, so we just require to have the stub
class require_package creates

Change-Id: I8ce1727d4ebf180d98d507fe0d41147887e3579b
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>