Move `utils.hash_file` -> `imagecache._hash_file`

`hash_file` is only used by `imagecache` so it doesn't make sense (yet) for it
to be included in utils.

Change-Id: I2f4d1deb03e4afceb911ef4b6d6ae6760ddca225
Merge "Move `utils.hash_file` -> `imagecache._hash_file`"