Patch Head Part's Valid Races to use merged keys.

Works (almost) the same way as it does for Worn Armor addons. As NPCs are imported, their races are registered into the importer which adds them to the form list. Ensures that the form list contains all of the races that are actually used by merged NPCs, and no other races.

Fixes #119

Focus.Apps.EasyNpc/Build/Pipeline/NpcFacesTask.cs

@@ -87,6 +87,8 @@ protected override Task<Result> Run(BuildSettings settings)
                         if (mergedHeadPart.HasValue)
                             record.HeadParts.Add(mergedHeadPart.Value);
                     }
+                    foreach (var headPart in record.HeadParts)
+                        patch.Importer.AddHeadPartRace(headPart, record.Race);
                     log.Debug("Importing hair color from {pluginName}", model.FaceOption.PluginName);
                     record.HairColor.SetTo(patch.Importer.Import(faceNpcRecord.HairColor, x => x.Colors));
                     log.Debug("Importing face texture from {pluginName}", model.FaceOption.PluginName);

Focus.Apps.EasyNpc/Build/Pipeline/RecordImporter.cs

@@ -50,6 +50,28 @@ public void AddArmorRace(
                     addon.AdditionalRaces.Add(actualRace.FormKey);
         }
 
+        public void AddFormListKey(IFormLinkGetter<IFormListGetter> formListLink, FormKey itemKey)
+        {
+            var formListKey = formListLink.FormKey;
+            if (formListKey.IsNull || formListKey.ModKey != mergedMod.ModKey)
+                return;
+            var formList = GetIfMerged<FormList>(formListKey);
+            if (formList is not null && !formList.Items.Contains(itemKey))
+                formList.Items.Add(itemKey);
+        }
+
+        public void AddHeadPartRace(IFormLinkGetter<IHeadPartGetter> headPartLink, IFormLinkGetter<IRaceGetter> raceLink)
+        {
+            if (raceLink.IsNull)
+                return;
+            var headPartKey = headPartLink.FormKey;
+            if (headPartKey.IsNull || headPartKey.ModKey != mergedMod.ModKey)
+                return;
+            var headPart = GetIfMerged<HeadPart>(headPartKey);
+            if (headPart is not null)
+                AddFormListKey(headPart.ValidRaces, raceLink.FormKey);
+        }
+
         public void AddMaster(string pluginName)
         {
             masters.Add(ModKey.FromNameAndExtension(pluginName));
@@ -140,6 +162,11 @@ private void ImportArtObjectDependencies(ArtObject art)
             log.Information("Finished processing cloned art object {FormKey} '{EditorId}'", art.EditorID, art.FormKey);
         }
 
+        private void ImportEmptyFormList(FormList formList)
+        {
+            formList.Items.Clear();
+        }
+
         private void ImportHeadPartDependencies(HeadPart headPart)
         {
             log.Debug(
@@ -148,6 +175,7 @@ private void ImportHeadPartDependencies(HeadPart headPart)
             headPart.Flags &= ~HeadPart.Flag.Playable;
             headPart.Color.SetTo(Import(headPart.Color, mergedMod.Colors));
             headPart.TextureSet.SetTo(Import(headPart.TextureSet, mergedMod.TextureSets));
+            headPart.ValidRaces.SetTo(Import(headPart.ValidRaces, mergedMod.FormLists, ImportEmptyFormList));
             ReplaceAlternateTextures(headPart.Model?.AlternateTextures);
             ReplaceList(headPart.ExtraParts, mergedMod.HeadParts, ImportHeadPartDependencies);
             log.Information(