[turbopack] standardize chunk entry names for webpack and postcss loaders (#94256)
### What?
Use the `chunk_path` api for computing entrypoint filepath names for our
node worker pool files. For postcss loaders ensure that the config
filepath is part of the module ident of the transformer.
### Why?
I was investigating a bug in reported in next 16.2.6
(https://vercel.slack.com/archives/C09GRCW5MQR/p1780088728756579) and
noticed that the chunk entry path construction logic was unusual. It
happens to be that https://github.com/vercel/next.js/pull/91338 fixed
the reported bug (just not released yet). This migrates all entry
construction to the standard form