Filter compatible modules by compatibility

[HebaruSan]

Problem

When you check the mod list checkbox to install a mod, the latest version is chosen regardless of compatibility:

Cause

In #2963 we renamed Registry.Available to Registry.CompatibleModules and rewrote it to share a persistent data structure with Registry.IncompatibleModules that stores all the compatible AvailableModules in one list and the incompatible ones in another list. Then all we have to do is return one CkanModule per AvailableModule in a list.

Previously we returned the latest CkanModule regardless of its compatibility. This meant that when a GUIMod got constructed for a given row, its Mod property could be set to an incompatible CkanModule, and this is what is used for the default version to install.

Changes

Now we pass the version criteria to AvailableModule.Latest() when generating the list of CkanModules, so the list will contain actually compatible modules. This will allow GUIMod to select the correct Mod value.
(And pair is renamed to avail since these are AvailableModule objects rather than the KeyValuePairs that we would commonly see when working with a dictionary.)

Fixes #2979.