Directory: Use Win32 find file functions

The previously used UCRT functions just add overhead without any benefit.

What I mean with overhead is, that around 76% of _wfindfirsts runtime is spent by converting between Win32 file times and UCRT time_ts, which we don't even use:

(Weight column is time in ms)

Improvements of CMakes runtime match with what WPA says:

Before:
  Time (mean ± σ):     18.451 s ±  0.065 s    [User: 0.003 s, System: 0.006 s]
  Range (min … max):   18.378 s … 18.602 s    10 runs
After:
  Time (mean ± σ):     18.189 s ±  0.056 s    [User: 0.000 s, System: 0.001 s]
  Range (min … max):   18.111 s … 18.300 s    10 runs