Tweak package selection input logic in Powershell install script

[trainmeditations]

Moved the output of the options to a function Loops input until a valid option is selected Used $j in local logic, export selection as $PKG_INDEX

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This change is 

[NiLuJe]

As you may have surmised, I have basically no idea how PS works ;p.

But this looks sound, thanks :).

[trainmeditations]

No problem, was just checking it out before running it, there's a couple of other tweaks I could take a look at too.