yarnpkg/yarn ? reverse-engineered prompt

Build me a JavaScript package manager like Yarn 1, with a simple command line experience for installing, adding, removing, and updating dependencies.

I want it to be fast, reliable, and secure, so it should cache downloaded packages, work offline when possible, use a lockfile so installs are repeatable on every machine, and verify packages with checksums before running them. It should also handle network failures gracefully and avoid downloading the same things over and over.

Please make the usual Yarn style commands feel familiar, and include support for flattening duplicate dependency versions when possible. If anything is unclear, look up the current docs online and follow the behavior people expect from Yarn classic.