Update cargo_metadata to 0.8.0

[ThomasdenH]

Make sure these boxes are checked! 📦✅

• You have the latest version of rustfmt installed

$ rustup component add rustfmt

• You ran cargo fmt on the code base before submitting
• You reference which issue is being closed in the PR text

✨✨ 😄 Thanks so much for contributing to wasm-pack! 😄 ✨✨

This updates cargo_metadata to 0.8.0. The semantics slightly changed, but other than that there's not a lot of difference. The error type is simplified, which means a conversion function is no longer needed.

I was hoping this would fix (it doesn't) #637, but unfortunately not.

[ashleygwilliams]

thank you very much!