Flatten Material config dictionary

We are using HdCreateTypedRetainedDataSource() to convert the
config VtDictionary to a Container Data Source. This function does
not handle nested attributes, so mtlx:version was not making it
into Hydra. This change flattens the attributes so that the
mtlx:version does make it to Hydra.

(Internal change: 2356523)

pxr/imaging/hd/materialNetwork2Interface.cpp

@@ -88,8 +88,9 @@ HdMaterialNetwork2Interface::GetMaterialConfigKeys() const
 VtValue
 HdMaterialNetwork2Interface::GetMaterialConfigValue(const TfToken& key) const
 {
-    if (auto result = _materialNetwork->config.GetValueAtPath(key.GetString())){
-        return *result;
+    const auto it = _materialNetwork->config.find(key.GetString());
+    if (it != _materialNetwork->config.end()) {
+        return it->second;
     }
     return VtValue();
 }

pxr/usdImaging/usdImaging/dataSourceMaterial.cpp

@@ -802,25 +802,23 @@ _BuildMaterial(
     }
 
     // Collect any 'config' on the Material prim
-    // Collect in to a VtDictionary to take advantage of SetValueAtPath for 
-    // nested namespaces in the attribute names.
-    VtDictionary configDict;
+    TfTokenVector names;
+    std::vector<HdDataSourceBaseHandle> values;
     for (const auto& prop : usdMat.GetPrim().GetPropertiesInNamespace(
             UsdImagingTokens->configPrefix)) {
         const auto& attr = prop.As<UsdAttribute>();
         if (!attr) {
             continue;
         }
 
-        std::string name = attr.GetName().GetString();
+        const std::string name = attr.GetName().GetString();
         std::pair<std::string, bool> result =
             SdfPath::StripPrefixNamespace(name, UsdImagingTokens->configPrefix);
-        name = result.first;
+        names.push_back(TfToken(result.first));
 
         VtValue value;
         attr.Get(&value);
-
-        configDict.SetValueAtPath(name, value);
+        values.push_back(HdCreateTypedRetainedDataSource(value));
     }
 
     HdContainerDataSourceHandle nodesDs = 
@@ -830,7 +828,8 @@ _BuildMaterial(
             nodeValues.data());
 
     HdContainerDataSourceHandle configDefaultContext =
-        HdUtils::ConvertVtDictionaryToContainerDS(configDict);
+        HdRetainedContainerDataSource::New(
+            names.size(), names.data(), values.data());
 
     return HdMaterialNetworkSchema::Builder()
         .SetNodes(nodesDs)

pxr/usdImaging/usdImaging/materialAdapter.cpp

@@ -396,7 +396,7 @@ UsdImagingMaterialAdapter::GetMaterialResource(UsdPrim const &prim,
         VtValue value;
         attr.Get(&value);
 
-        configDict.SetValueAtPath(name, value);
+        configDict.insert({name, value});
     }
     networkMap.config = configDict;