Randomize package downloads

This is the core groundwork and essential changes to widen the use of mirrors, and ultimately let there be a lot more control over how mirrors can be used, whether that be user preferences like #1696 (closed) or algorithms for choosing the closest/fastest mirror. It should also clean up the related code a bit and use more descriptive naming. More details in the commit messages.

I'll tag an alpha release once this is merged, so we can get testing going.

review please, if this breaks things, it means package downloads fail: @Bubu @IzzySoft @Matrixcoffee @uniqx @cde