Fix duplicate key exception in CompatibilitySorter

[HebaruSan]

Problem

The metadata validator threw a duplicate key exception while checking KSP-CKAN/NetKAN#9800:

https://github.com/KSP-CKAN/NetKAN/actions/runs/6207962451/job/16853654203?pr=9800

  Unhandled Exception:
  System.ArgumentException: An item with the same key has already been added. Key: [kOS-EVA, CKAN.AvailableModule]
    at System.Collections.Generic.TreeSet`1[T].AddIfNotPresent (T item) [0x0001a] in <33b19a7ad5234d94abf4fd9b47566616>:0 
    at System.Collections.Generic.SortedSet`1[T].Add (T item) [0x00000] in <33b19a7ad5234d94abf4fd9b47566616>:0 
    at System.Collections.Generic.SortedDictionary`2[TKey,TValue].Add (TKey key, TValue value) [0x00020] in <33b19a7ad5234d94abf4fd9b47566616>:0 
    at CKAN.CompatibilitySorter.PartitionModules (System.Collections.Generic.IEnumerable`1[T] dicts, System.Collections.Generic.Dictionary`2[TKey,TValue] providers) [0x00114] in <f9f0ffbfa5e14bc18bf2f56116c5f6c1>:0 
    at CKAN.CompatibilitySorter..ctor (CKAN.Versioning.GameVersionCriteria crit, System.Collections.Generic.IEnumerable`1[T] available, System.Collections.Generic.Dictionary`2[TKey,TValue] providers, System.Collections.Generic.Dictionary`2[TKey,TValue] installed, System.Collections.Generic.HashSet`1[T] dlls, System.Collections.Generic.IDictionary`2[TKey,TValue] dlc) [0x0005b] in <f9f0ffbfa5e14bc18bf2f56116c5f6c1>:0 

Cause

The CompatibilitySorter from #2963 was updated to handle multiple source repos in #3904, but multiple AvailableModules with the same identifier weren't covered. This is essential for the metadata validator.

Changes

• Now if the repositories contain multiple AvailableModules with the same identifier, the CompatibilitySorter merges them into one new AvailableModule containing the combined list of CkanModules; if there are multiple CkanModules with the same identifier and version, the module from the higher priority repo (defined as higher up in the list, so a lower number) takes precedence
• A test is added to exercise this case, which fails without the fix and passes with it
• AvailableModule.Remove wasn't being used and is deleted
  AvailableModule.Add is only being used internally so is now private

We may have to update the metadata validator after this to set the priority of the repositories to get the right modules to load; I think we were previously skating by on the coincidence that default comes before local alphabetically.

[techman83]

Looks great, and extra tests 😍 thanks @HebaruSan !