correctly parse package.json to determine existing dependencies

[ZabelTech]

Make sure these boxes are checked! 📦✅

• [ x ] You have the latest version of rustfmt installed

$ rustup component add rustfmt

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

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

this fixes the immediate problem as mentioned in https://github.com/rustwasm/wasm-pack/issues/1420