Summary:
This is a [complicated topic](https://yarnpkg.com/getting-started/migration#switching-to-plugnplay). In short, Yarn can now replace `node_modules` with a virtual filesystem introspecting `.zip` files. This has many benefits, but it takes some work to enable.

Prerequisites:
- D8919: Namespaced absolute imports
- Changed `node_modules` references out for `require.resolve` and `yarn run ...`
- Made other references more explicit / less ambiguous
- Made some dependencies explicit instead of transitive

Enabling Plug n' Play (PnP):
- `.yarnrc.yml` was updated to actually turn PnP on.
- `yarn install`
- This created `.pnp.js` and deleted `node_modules` (in favor of `.yarn/cache`).

Supporting the change:
- Updated ignore files, lint files, Bazel files, etc.
- Enabled [Yarn's SDK](https://yarnpkg.com/getting-started/editor-sdks) to support VS Code go-to-definition
- Rewrote the Python license checker to be in JS, so that `yarn pnpify` could trick it into ...