Fix InitRxUI to account for order of passed registration namespaces (#3887)

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**What kind of change does this PR introduce?**
<!-- Bug fix, feature, docs update, ... -->
Bug fix?


**What is the current behavior?**
<!-- You can also link to an open issue here. -->
Originally the `InitializeReactiveUI` function didn't account for order
of passed registration, because the allowed namespace dictionary was
enumerated instead of the array (which is kind of backwards if you think
about it).


**What is the new behavior?**
<!-- If this is a feature change -->
Order of the namespaces is preserved when registering.


**What might this PR break?**
Multi-platform projects that rely on current non-determinant order may
break because of different order of registrations.


**Please check if the PR fulfills these requirements**
- [ ] Tests for the changes have been added (for bug fixes / features)
- [ ] Docs have been added / updated (for bug fixes / features)

**Other information**:
Not sure on what kind of tests/docs are expected, so I am leaving it as
draft for now, feel free to undraft if you feel this is OK.

Co-authored-by: Glenn <[email protected]>

Author: Metadorius

src/ReactiveUI/Mixins/DependencyResolverMixins.cs

@@ -58,9 +58,9 @@ public static void InitializeReactiveUI(this IMutableDependencyResolver resolver
         }
 
         var extraNs =
-            possibleNamespaces
-                .Where(kvp => registrationNamespaces.Contains(kvp.Key))
-                .Select(kvp => kvp.Value)
+            registrationNamespaces
+                .Where(ns => possibleNamespaces.ContainsKey(ns))
+                .Select(ns => possibleNamespaces[ns])
                 .ToArray();
 
         // Set up the built-in registration