perf(fs): normalize path only once in fs.dir

Re-normalizing a path after a `joinpath` isn't necessary. Calling
`normalize` on each child directory had quite a bit of impact when
traversing a large directory.

A simple test showed:

Before: ~144ms
After: ~80ms

running the following logic against a dir with 4367 child folders and
25826 files:

    local files = {}
    local start = uv.hrtime()
    for name, type in vim.fs.dir(path, { depth = max_depth }) do
      table.insert(files, { name, type })
    end
    local duration = uv.hrtime() - start

Relates to https://github.com/neovim/neovim/issues/23291

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