Auto merge of #2359 - matklad:metadata-no-deps, r=alexcrichton

closes #2356. The output is not exactly the same as with `read-manifest`, because I wanted to include the `version` field.

Tests and implementation is a copy-paste from `read-manifest` except that [this line](https://github.com/rust-lang/cargo/blob/master/src/bin/read_manifest.rs#L36) is omited, because `root_package` [does update anyway](https://github.com/rust-lang/cargo/blob/master/src/cargo/sources/path.rs#L51).