Issue description

I've been working on the new file set library in recent months, which allows selecting local eval-time paths to add to the store. Notably this library does not support handling of store paths, see #264537 for why not.

However, it would be very doable to have a simple function for doing arbitrary transformations over store paths with a command. I can imagine an interface like this:

pkgs.transformStorePath {
  path = pkgs.hello;
  command = ''
    rm -rf share
    mv $out/bin/{hello,hallo}
  '';
}

I can also imagine there being alternate more convenient approaches, suggestions welcome.

Ping @roberth @fricklerhandwerk

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