Closes#17339
This PR fixes an issue that caused changes to `@import`-ed CSS files to
no longer rebuild the stylesheet after a change was made to a template
file.
The change in the template file causes a fast-path in the Vite plugin
now after changes in 4.0.8: _partial rebuilds_. For that branch we do
not need to re-evaluate your input CSS since we know only the candidate
list changed. However, we still need to emit all build dependencies as
via `addWatchFile(…)`, otherwise Vite will not correctly register
updates for these dependencies anymore.
## Test plan
- Updated the kitchen-sink Vite update tests to ensure that an
`@import`-ed CSS file can be updated even after a partial rebuild.
- Ensure this works in our Vite playground
fac8f7df62