Turbopack: fix duplicate unsupported edge import modules (#78236)
1. Add the import in question to the module ident: `[project]/test/e2e/on-request-error/isr (unsupported edge import fs)`
2. Wrap the source creation into a separate turbotask, so that `fs` and `fs/promises` get the same Source cell (instead of two different Sources with the same ident)