lookaside: Move handling of file hashing
    
    The only time we need to compute the hash of a source file is when we
    interact with the lookaside cache, so it makes sense to have it there,
    and remove some clutter from the main module.
    
    However, the Commands._hash_file might be used by downstream, so let's
    keep it for a while, make it use the new lookaside module, and emit a
    deprecation warning.