Summary:
The public_autocargo is for external builds, not internal ones as they contain
references to git repository, while the non-public_autocargo refers to path in
fbcode. In Sandcastle, git repositories can fail to be accessed due to lack fo
proxies, thus we shouldn't access them.

Reviewed By: kmancini

Differential Revision: D33907864

fbshipit-source-id: 3f2006607676cebd92201560365d179231214db4