Support package self-referencing using its name

[PaperStrike]

Node.js allows one to self-reference the package by the package's name:
https://nodejs.org/api/packages.html#self-referencing-a-package-using-its-name

This is helpful for writing tests that reference the package instead of directly to the files, by that we can avoid import paths like ../../../src/xxx, and test if the exports field is in the correct format to be consumed.

Thank you!

[evanw]

I was wondering if/when someone was going to ask for this. You are the first! I can look into it.