Make Blobstore::Client#file a public method

* used by the buildpack bits controller, hopefully will be refactored to use
  the new blob pattern.

Signed-off-by: Kai Zhang <kaizhang@cn.ibm.com>
Refactor buildpack_bits_controller to use Blob instead of file

Signed-off-by: Kai Zhang <kaizhang@cn.ibm.com>
Fix typo in buildpack_bits_controller

Signed-off-by: Kai Zhang <kaizhang@cn.ibm.com>