normalize the scratch path
normalize the scratch path

Summary:
The scratch path is used as part of the hash computation for each project.  We
need to make sure this path is always normalized to ensure that we compute the
hashes consistently.

Reviewed By: chadaustin

Differential Revision: D16354624

fbshipit-source-id: 39b5362620bdc247cd7e7f1333dac319b354dc6f

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly