Snowpack: Support Yarn v2 (berry) with config extends

[FredKSchott]

Follow up from https://github.com/yarnpkg/berry/issues/1487

Right now, we use "extends" to extend a configuration object from another. If we extend from a package, it is better to resolve all imported packages from that location instead of from the current working directory. See this comment for an explanation of how our current implementation can be an issue: https://github.com/yarnpkg/berry/issues/1487#issuecomment-644577403

Instead, we need to resolve any plugins relative from the location of the extended config, most likely before we merge with the root user config. This is different from what we do today, where the object is merged before the config is loaded.

[JennieJi]

@FredKSchott I think this can be closed?

[FredKSchott]

Yes, I think so!

[FredKSchott]

Thanks again for your help on this!