yarnpkg executable no longer available after running buildpack

The old `heroku/nodejs-corepack` buildpack used to expose the `yarnpkg` executable in the `shim` layer:

```
$ ls -l /layers/heroku_nodejs-corepack/shim/bin
total 0
lrwxrwxrwx. 1 cnb cnb 73 Jan 1 1980 yarn -> ../../../heroku_nodejs-engine/dist/lib/node_modules/corepack/dist/yarn.js
lrwxrwxrwx. 1 cnb cnb 76 Jan 1 1980 yarnpkg -> ../../../heroku_nodejs-engine/dist/lib/node_modules/corepack/dist/yarnpkg.js
```

When updating to the latest `buildpacks-nodejs` version, we noticed that corepack is no longer used, and it now creates a `yarn_vendored` layer that only includes the `yarn` executable:

```
$ ls -l /layers/heroku_nodejs/yarn_vendored/bin
total 4
-rwxr-xr-x. 1 cnb cnb 73 Jan 1 1980 yarn
```

This broke some of our old apps that still tried to find the `yarnpkg` executable.

Would it be okay to also add an executable for `yarnpkg` in the `yarn_vendored` layer? I can work on a quick PR if that's the case.

Start by tracing the buildpack code that creates the yarn_vendored layer and its bin contents. Compare the current layer with the old shim behavior described here; done means the layer exposes both yarn and yarnpkg without breaking the existing yarn executable.

Written by the indexing model from the issue text.