fix resolving package self references

[devongovett]

The Node ESM resolution algorithm has a special case for package self references, i.e. packages importing themselves. Usually, these can be resolved as normal, by resolving from node_modules/foo/bar.js up to node_modules/foo/package.json. However, if the module is not actually inside a node_modules directory this doesn't work. In particular, Pnpm uses a web of symlinks into a .pnpm directory, and does not symlink the module itself into its own local node_modules folder. Therefore, we need to implement this special case for self references.