Problem:
61e99217e6 replaced usages of `vim.fn`. This duplicates non-trivial
logic and may have introduced bugs like 38e38d1b40.
Later on, b02eeb6a72 graduated `fnamemodify` to `fast`, so avoiding it
in `vim.fs` is no longer necessary.
Solution:
Use `vim.fn` to deduplicate `vim.fs.dirname()` and `vim.fs.basename()`.
Note: the "nvim -l" test-runner switch from the original PR (#30483) is
already done by 9432e6c1e2 (#39676).
c7f5f44b60