Detect changes to replaced_by

[HebaruSan]

Problem

During KSP-CKAN/NetKAN#10108, we noticed that replaced_by is a bit janky.

Causes

• We check for the replaced_by property in the installed module, not the available module, so if users install a module and then later we add replaced_by to migrate them to a new identifier, they won't see the replace checkbox unless they reinstall the mod to get the new metadata.
• Compounding this, when we decide whether to prompt for reinstallation, replaced_by isn't checked. So the user would have to reinstall manually, which mostly defeats the purpose.

Changes

Now if you add replaced_by to a module, users who already have it installed will be prompted to reinstall it with the upgrade checkbox, after which they'll be prompted to replace it with the replace checkbox.

replaced_by could use more streamlining after this, but this will at least make it baseline usable.