Fix CSS duplication related problems (#50406)
This PR fixes a couple of categories of CSS issues in App Router, that
come from the same root cause.
### 1. Duplicated styles being loaded in different layers
This issue has been described in
https://github.com/vanilla-extract-css/vanilla-extract/issues/1088#issuecomment-1563931144.
If a CSS module (or a global CSS) is referenced in multiple layers (e.g.
a layout and a page), it will be bundled into multiple CSS assets
because each layer is considered as a separate entry.
<img width="1141" alt="CleanShot-2023-05-26-GoB9Rhcs@2x"
src="https://github.com/vercel/next.js/assets/3676859/8e0f5346-ee64-4553-950a-7fb44f769efc">
As explained in that issue, we have to bundle all CSS modules into one
chunk to avoid a big number of requests.
### 2. CSS ordering issues (conflicts)
This is likely causing https://github.com/vercel/next.js/issues/48120.
When the layer-based bundling and ordering logic applies to CSS, it can
potentially cause non-deterministic order. In this example, button A in
the layout should be in blue. However when button B is imported by the
page, button A becomes red. This is an inconsistent experience and can
be hard to debug and fix.
<img width="1090" alt="CleanShot-2023-05-26-Ar4MN5rP@2x"
src="https://github.com/vercel/next.js/assets/3676859/4328d5d7-23af-4c42-bedf-30f8f062d96a">
