turbopack-css: fix rule duplication (#6778)
This fixes an issue in css chunk generation where rules could be
duplicated, which was likely introduced in the chunking refactor (a
series of PRs starting with #6104).
CSS chunk generation had special logic that would walk the module graph
on its own and include code. Combined with the topological list of
modules provided by chunking, this unnecessarily included duplicate
rules in incorrect order.
Test Plan: Added and updated snapshot tests.
Closes PACK-2141
