Auto merge of #2201 - matklad:serialized-dep, r=alexcrichton

Auto merge of #2201 - matklad:serialized-dep, r=alexcrichton

This hides `SerializedDependency` from general public, as requested [here](https://github.com/rust-lang/cargo/pull/1434#issuecomment-97886950). It also hides `SerializedManifest` which was (wrongly?) exposed.

This is required for #2196. I want to move in small steps this time, hence the separate PR.

Technically this break backwards compatibility, because `SerializedDependency` and `SerializedManifest` were public (`SerializedPackage` was private however). Are such changes allowed in cargo?