Use mklink under Windows

In order to support in-place importing on Windows, mklink
rather than ln must be used. Rather than create new commands
like `--transfer=mklink`, the same flags (`ln` and `ln_s`)
are being used.


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Notes: opening to simplify testing by Anatole.
Noted: I likely won't push here, but in a cleaned up PR with fewer commits if there's to be any chance of getting this into 5.0.3