Auto merge of #5360 - euclio:metadata, r=alexcrichton

include package metadata in `cargo metadata`

Fixes #4819.

All-in-all a pretty small change. I'm a little concerned about threading the `toml::Value` all the way through however, should I make another type? Also, cloning the metadata value initially is maybe not ideal, since it's technically accessible through the original manifest. I figured this way was cleaner.