Add fs::createPath function

Summary:
add fs::creatPath function and make sure that all outputs and depfile of a rule
have their directory created.

Test Plan:
Build. Then remove an output directory. Build again. Check that
falcon re-created the directory that was removed.

Reviewers: NicolasDP

Reviewed By: NicolasDP

Differential Revision: http://91.121.199.76/D60