Fixes#16732
If we can not get the mtime from a file, chances are that the resource
is a virtual module. This is perfectly legit and we can fall back to
what we did before the changes in `4.0.8` (which is to rebuild the root
every time a change contains a dependency like that).
## Test plan
Added a test to mimic the setup from the repor in #16732. Also ensured
the repro now passes:
<img width="1278" alt="Screenshot 2025-02-24 at 17 29 38"
src="https://github.com/user-attachments/assets/d111273d-579f-44c2-82f5-aa32d6a1879a"
/>
Note that importing virtual modules directly in CSS does not work as the
resolver we use does not resolve against the Vite runtime it seems. This
is unrelated to the regression added in `4.0.8` though and something to
look into in the future.
