Entry point cannot be marked as external when making node_modules external for node, on Windows.

When I run build I get "The entry point cannot be marked as external" with this being the resolver plugin, only on windows machines, it works fine for wsl and posix.

```ts
{
name: 'make-all-packages-external',
setup(build) {
// iife files should be fully bundled.
if (build.initialOptions.outExtension?.['.js'].startsWith('.iife.js')) {
const filter = /^vue$/
build.onResolve({ filter }, () => {
return {
path: 'Vue',
external: true,
}
})
} else {
// It fails here!
const filter = /^[^.\/]|^\.[^.\/]|^\.\.[^\/]/ // Must not start with "/" or "./" or "../"
build.onResolve({ filter }, args => ({ path: args.path, external: true }))
}
},
},
```

Start by reproducing the supplied configuration on Windows and compare it with WSL or a POSIX system. Inspect the resolver plugin's build.onResolve handler and its path filter; done means node_modules dependencies can be marked external without the entry-point error while the existing IIFE behavior remains intact.

Written by the indexing model from the issue text.