Write v3 plugins that depend on deep imports to @yarnpkg/core

[kherock]

I have a Yarn 2 plugin that I'm trying to upgrade to for version 3, but it seems like the builder/plugin loader has changed the way plugin dependencies are available at runtime. The plugin itself builds fine, but at runtime, the cli tells me

This plugin cannot access the package referenced via @yarnpkg/core/lib/MultiFetcher which is neither a builtin, nor an exposed entry

I realize writing plugins using deep imports probably wasn't an intended feature, but I think it should still be supportable. My code is mainly just trying to reuse some of the core's existing implementations without copy-pasting. Rather than complain at runtime, I think it would be nice to have the builder include (and perhaps warn about) the relevant tree-shaken internal code into the plugin bundle. I'm not familiar with the builder's structure well enough to figure what I might need to change, or if esbuild would even support something like this.