[Feature] migrate offline cache from v1 to v2

[IanVS]

• I'd be willing to implement this feature (contributing guide)
• This feature is important to have in this repository; a contrib plugin wouldn't do

Describe the user story

I am attempting to migrate from yarn v1 to v2. My company has been using the yarn-offline-mirror setting, and checking in the .tar.gz files. I would like to migrate to yarn v2 without re-downloading all of my dependencies from the internet, losing confidence that the files have not changed since originally downloaded.

Describe the solution you'd like

I would like to tell yarn to use my existing offline files, or that it should convert them into the new format rather than re-downloading them.

Describe the drawbacks of your solution

I can't think of any.

Describe alternatives you've considered

I'm not sure how this could be implemented, whether it could be a flag to the command line to tell yarn berry where to find legacy offline mirror files, or something else.

[Lunarequest]

Downstream building applications in flatpaks requires a offline cache, this would be a massive help for us

[jhudsoncedaron]

Just found out we're in the same boat. We never upgraded off of yarn 1 until another developer tried to switch over and couldn't follow the setup instructions for the project because of yarn version.

We don't trust the packages we use to not just go away on the upstream server sometime in the future.