Handle depfiles in local caching

Summary:
We now have a second hash which makes it possible to retrieve the implicit
dependencies. When a file is modified, we use this hash to determine if the
implicit dependencies have changed.

Test Plan: Run the tests

Reviewers: NicolasDP

Reviewed By: NicolasDP

Subscribers: adrienconrath

Differential Revision: http://91.121.199.76/D56