use `_readdirx` for `walkdir` (#53545)
On a M2 Mac there is some benefit, but assumed to be much greater on
slower filesystems.
```
# master
julia> @btime collect(walkdir(expanduser("~/Downloads")));
380.086 ms (310696 allocations: 25.29 MiB)
# This PR
julia> @btime collect(walkdir(expanduser("~/Downloads")));
289.747 ms (103300 allocations: 7.50 MiB)
```
The implementations appear to produce the same result
```
julia> collect(walkdir(expanduser("~/Downloads"))) == collect(walkdirx(expanduser("~/Downloads")))
true
```
