Rename package api to dgraph

Moved from GitHub dgo/44

Posted by Swoorup:

Why is the package for api.proto api? Shouldn’t it be specific to dgraph, maybe dgraph_api or simply dgraph?

Using api gives bit nonsensical usage like here, from grpc generated code.
https://github.com/Swoorup/dgraph-client-rs/blob/master/examples/dgraph_tuto01/main.rs#L40