Watch.zig: fixes for windows implementation

[jayrod246]

Using --watch I noticed a couple of issues with my initial attempt.

1. The index I used as 'completion key' was not stable over time, when directories are being added/removed the key no longer corresponds with the intended dir.
2. There exists a race condition in which we receive a completion notification for a directory that was removed.

My solution is to generate a key value and associate it with each Directory.