fix: ensure local files mapped to yarn virtuals are watched

[mrginglymus]

Description

• Fixes Yarn virtual packages/workspaces not watched in pnp #15809, but probably not in the best way

There's a somewhat minimal repro here: https://github.com/mrginglymus/vitest-pnp though the focus of that is worrying about the coverage reporting on vitest.

The specific problem is this:

1. Using Yarn PnP
2. With a workspace package with peer dependencies
3. With source files in that workspace package
4. Imported from files via package name from the root package

Then files are reported as being in <cwd>/.yarn/__virtual__/my-package....

I've failed to exactly follow how files are added to the watcher, but what I can see is that ensureWatchedFile bails if the file considered 'out of root', as presumably it would otherwise be watched.

However, these virtual files are not watched; I don't know how the initial watchlist is populated, but I could envisage it either having an exclusion of directories with a leading ., or scanning the on-disk filesystem (which would not include the virtual files).

Either way, what 'fixes' this for me locally is to either change the root (to trick ensureWatchedFile) or apply this fix which considers virtual files to be out of root.

[mrginglymus]

Actually this still doesn't help - even if the virtual file is watched, sometimes the on-disk file will be reported as changed but will have been registered in the module graph as a virtual file so no change is triggered...