Providers are a new concept for Blueprint, based on providers in Bazel:
https://docs.bazel.build/versions/master/skylark/rules.html#providers
Providers aim to simplify the interaction between modules by replacing
type asserting to arbitrary interfaces with requesting optional data
objects from modules. This will also move Blueprint closer to supporting
incremental analysis by serializing the providers and only rerunning
the analysis phase on modules whose inputs have changed.
Change-Id: I39f5f78b372412a7dbf151ceccb3f917f6c874bf
