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
```

base/file.jl

@@ -1089,18 +1089,16 @@ function walkdir(root; topdown=true, follow_symlinks=false, onerror=throw)
  end
  return
  end
- content = tryf(readdir, root)
- content === nothing && return
- dirs = Vector{eltype(content)}()
- files = Vector{eltype(content)}()
- for name in content
- path = joinpath(root, name)
-
+ entries = tryf(_readdirx, root)
+ entries === nothing && return
+ dirs = Vector{String}()
+ files = Vector{String}()
+ for entry in entries
  # If we're not following symlinks, then treat all symlinks as files
- if (!follow_symlinks && something(tryf(islink, path), true)) || !something(tryf(isdir, path), false)
- push!(files, name)
+ if (!follow_symlinks && something(tryf(islink, entry), true)) || !something(tryf(isdir, entry), false)
+ push!(files, entry.name)
  else
- push!(dirs, name)
+ push!(dirs, entry.name)
  end
  end