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.
There is a `PlatformAssumptions.isWindows()` in `blitz/test`. Just a comment/thought though, it certainly needn't block this PR.